Need some hints on threading the lower threads, i find it very difficult, sometimes it works and others it doesn't, then for no reason at all it goes through and everything seems ok, but it is very frustrating.

Um-hmm. Been there, done that! Hopefully, you have a manual (I'm not able to find one online).

Here are a couple of hints:

Remove all the thread from the machine.
Recommend using the colored threads that match the machine's guide--will help you figure out which thread is misbehaving.
ALWAYS RAISE the presser foot and thread each in the correct order! Check your manual, but most machines need to be threaded #3 (upper looper), #4 (lower looper), then the needles.
With each thread, grasp the thread below the thread tree and also below the tension disk and give a gentle tug. Sometimes the thread will pop when it seats in the tension unit.
The lower looper thread tail should drape OVER the top of the upper looper and thread, then sweep all threads toward the back of the machine and under the presser foot.

You might try contacting the Necchi Support service.

Difficult to say. There could be something wrong with the tension unit itself that would require a technician.

Just a couple of hints:
when threading the machine, ALWAYS have the presser foot in the RAISED position.

When threading through the tension unit, grasp the thread below the thread tree and grab the other end below the tension unit, give the thread an extra little tug to make sure it is seated completely. Sometimes I can hear the thread "pop" when it seats.

You can also try cleaning the tension unit. I dip a piece of perl cotton (about 12") into rubbing alcohol and squeeze out the excess. Then raise the presser foot, decrease the tension all the way to zero. Then, very gently, pull the perl cotton DOWN through the tension unit. NEVER pull UP! Do that a few times. If there is any gunk or thread pieces stuck, hopefully, it will be removed.

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You need 2 spools of thread. Thread one needle all the way through the path. Then thread the other needle. NOTE: Some machines (newer models mostly) will have 2 separate tension units for separate threads. On older models, you will just have to use the same path for both threads. The reason you thread each thread separately is to keep the threads from becoming twisted in the path and causing knots that will affect the stitch quality and tension.

HINT: ALWAYS RAISE the presser foot when threading the top thread on your machine.

Looper thread breakage occurs frequently when the thread path is incorrect or threaded out of order. Most machine must thread the upper looper first, then the lower looper. The lower looper thread ends "draped over" the upper looper in the last step before being pulled toward the back of the machine and under the presser foot.

Another common cause is tension that is too tight.

VERIFY the thread path is correct for every thread!!!! Missing a thread guide or looping around a guide will throw your machine for a "loop."

Another hint, when threading, ALWAYS RAISE the presser foot throughout the process and be sure to give each thread an "extra little tug" when going through the tension to make sure it is seated completely. Sometimes, the thread will even "pop" when it seats. When a thread break, checks that the thread has not "jumped" out of a thread guide or the tension disk.

If the thread breaks, it is usually less frustrating (in the long run) if you remove ALL the thread and start over from the beginning. Trying to snake a broken thread out of order through the sequence is frequently 99% UNsuccessful!!! It takes a lot of familiarity with your machine to be successful threading out of order, but there is still a lot of room for failure!!!

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https://answers.yahoo.com/question/index?qid=20120705022414AAvIeRH

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Difficult to assess without a make & model.

The machine has a tensioner for the bobbin winder. Make sure that you are threading that tensioner before taking it to the bobbin winder. Also, sometimes the thread should make a complete loop around that tensioner (consult your owner's manual). Check that the thread spool is spinning the correct direction--most often the thread spool turns counter-clockwise on the spool pin. Running the thread the wrong direction could cause thread kinks or the thread plies to separate. Slow down the speed that you wind the bobbin. Fast speeds will cause the thread to stretch. Plus, if something goes haywire, you won't have a bobbin bouncing or thread flying all over the sewing room before you can stop the machine. Frequently, the bobbin will fill more in one area of the bobbin. You can gently guide the thread by pushing it up or down lightly with a pencil tip or other object to even the thread distribution on the bobbin.

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There are several reasons.

1. The thread spool slit that you put the thread through to keep the spool from unwinding when it\'s not being used could be catching the thread as it unwinds. Check to be sure that the thread can be pulled from the spool without snagging.

2. One of the guides that the thread goes through may have a rough edge, or, more likely, is covered in thread lint and gunk, causing the thread to be pulled. It doesn\'t take much to snag thread. Use a q-tip and alochol, clean all of the threading points on the machine.

3. The tensioner is another possible hang up. Make sure that the tension is not too tight for the type of thread or fabric that you are using.

4. The needle may have a burr or rough spot on it. Replace the needle to see if the problem continues.

5. The bobbin case is the last place of concern. there may be a lot of thread lint, or the bobbin case is not seated properly. Sometimes, the thread hood under the bobbin case needs cleaning. Following the cleaning instructions in your manual, remove the bobbin case and thoroughly clean in there.

6. Finally, check in your manual to see if anything need oiling.

You might take a look on ebay item with bernina 700d/800dl instruction manual

Go to Consew's web site and look under resources for copy of instruction book. Its in a Pdf file you can print.

The most likely cause is that the top thread is not under tension.

You need to review your threading up; raise the presser foot first so that the tension device is open. Then thread from the thread spool to the needle as per your manual, paying attention to the thread take up lever, and the tension device and ensuring the thread goes into all the thread eyes on the path.

A good way to check if the top tension device is working is the following:
Turn tension to number 4, thread up to just before the needle bar thread guides, then lower the pressure foot to engage the tension and pull downwards on the thread. It should feel very firm and be difficult to pull. If this is not happening at all and you can see that the thread has seated into the tension device fully, then the device itself is faulty and it will need to be serviced.

The top tension pulls the upper thread up to the top of the fabric. The bobbin tension pulls the lower thread to the bottom of the fabric.

An even stitch is a tug of war between the two.

If it looks good on the to and the bottom looks loopy, the bottom is dominating and the upper tension needs to be increased to pull the knots up.

If the thread on the top looks like a straight line, it is too tight on the top or too loose on the bottom.

Unless the bobbin case tension has been adjusted, the problem is usually on the top.