Needle continually unthreads itself

Your machine has most likely seized due to lack of regular maintenance. The following method works on most mechanical machines. I've used it on a Pfaff 260 and several older Berninas. Oil & Heat!

Wheel on right side that controls the needle up and down is very tight...

How can fix stuck needle bar on my Bernina 730


PFAFF tipmatic tiptronic 1151 Instruction Book


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Ok... so now that you have the length of thread the proper length to start a seam. Are you holding the threads when you start to sew?

Is your needle the proper size for your thread and project?...
Also check the throat plate (where the needle goes through to the bobbin) is it in good shape... or does it show stab marks from the needle? If it shows stab marks.... it could be damaged an may need replaced.

I advise putting your project away and only using scrap pieces to sew on until you get this issue resolved.

try changing the needle--make sure to use every thread guide
try a different thread
check for any burrs on the needle plate or on the bobbin case
clean under the needle plate & in the bobbin area for lint or loose
threads

make sure to keep long thread tail for the top & bobbin
of 4 to 6 inches & pulled behind the needle

you need long thread tails, of 3 to 6 inches for the top & bobbin thread so the needle doesn't come unthreaded,
also pull the thread behind the needle before you start to sew

Top tension is insufficient, so probably your top tension dial is either turned right down, gunked up or the thread isn't into it properly. Start by unthreading top thread, raising pressure foot, turn tension dial to 5 and thread again to just after the dial, then lower the pressure foot and pull the thread, is it very firm to tug? If there is no tension on it, then the dial is faulty.

If there is tension, then try sewing again, maybe it was just threading up issue.

If you can access the metal discs that make up the tension device, then turn down to zero and "floss" between them with the selvage edge of a clean piece of cotton fabric and see if any gunk comes out. Many black and dark threads will shed dye particles and if you have used cheap cotton this will shed too, gunking up the tension. Good practice to clean it out regularly.

first let's deal with the needle becoming unthreaded, Keeping the needle threaded is easy, always keep thread tails of 3 to 6 inches on the top thread & bobbin at all times, & make sure the thread tail are pulled behind the needle before you start to sew,, this will ensure that the thread will not come out of the needle when you start to sew.

you mention the fabric is thin,, do you have a needle that's right for the fabric your sewing?
thin fabric needs a different needle than blue jeans fabric, check your top tension & see what the setting is on, usually 3 to 5 is good.

when the needle & thread are right for the fabric then the sewing is good.

also clean under the needle plate & bobbin area for loose threads or lint

also it's a possibility that the machine could be out of time & would need to be serviced by a sewing machine repair shop-as they have the tools for the job

check to see if the thread has come off the thread take up lever

check in the needle plate to see if the feed dogs are up or down, if they are down then the knob or switch need to be changed to the up position
if the feed dogs are up then it's possible that they need to be checked
because the feed dogs are what moves the fabric against the presser foot as the machine sews

make sure to have 3 to 6 inch thread tail on top thread & bobbin & pull the thread behind the needle before starting to sew that way the needle does not become unthreaded

and make sure the presser foot in the up position when thread

if none of these suggesions help then it's possible that the machine could be out of time & would have to be serviced by a sewing machine repair tech that has the tools to check the machine

firstly are you using a regular twin needle on a knit fabric? this could cause skipped stitches.
is the twin needle quite old and possibly blunt?

Is the Hobbymatic a model that requires a slanted twin needle, there are two types of twin needles as the computerised machines need the offset style. You can distinguish between them by the plastic block on the needle shaft where the twin needle points are set, either red or a deep brown colour.

If none of the above apply, then it is possible that the timing sequence is slightly off so that one of the needle's thread is not getting caught during the stitching.

Timing is an adjustment that I leave to the technician to address. But if you want to have a go yourself there may be some videos on you-tube for your model and how to make this adjustment to it.

this is a known issue with the pfaff.....i have just bought the machine and it has thread uptake issues...thread tension issues and it comes unthreaded by itself often...leaving globs of thread on the bottom stuck in the feed dogs and under the throat plate.....then the stitches are not properly balanced because the tension gets too loose on the bobbin thread....very disappointed to find out it is a known issue with all the viking husqvarna made machines...which are pfaff, janome and viking.....sux.