The thread hangs up inside the bobbin holder - Singer 2517 Mechanical Sewing Machine

Only thing is to remove the bobbin, bobbin holder if you can, the feed dog cover plate, and then, using a magnifying lamp and a fine pointed pair of tweezers, try and locate the bits of cotton and pull gently out while rotating the hand wheel slowly, rocking it back and forth. Use an air blaster can to blow fluff out, clean, and oil. Including inside the bobbin case.

make sure tat there about 6" of bobbin thread hanging out after you put the bobbin back in the bobbin holder
increase the needle thread tension up more thread tension should be a slight drag as you pull the thread out to thread the needle

The needle thread breaks
1. The needle thread is not threaded properly
2. The needle thread tension is too tight
3.the needle is bent or blunt
4. The needle is incorrectly inserted into the needle holder
5. The needle thread and the bobbin thread are not set under the presser foot when starting sewing
6. The threads were not drawn to the rear after sewing
7. The thread is either too heavy or too fine for the needle
Bobbin thread breaks
1. The bobbin thread is not threaded properly in the bobbin holder
2. Lint has collected in the bobbin holder
3. The bobbin is damaged and doesn't turn smoothly

1. TAKE THE BOBBIN AREA APART. BLOW IT OUT AND USE A QTIP TO CLEAN IT BETTER.
2. OIL THE RACE AREA (UNDER THE BOBBIN CARRIER) WITH ONE DROP OF SEWING MACHINE OIL.
3. HANG YOUR CARRIER WITH THE BOBBIN INSERTED WITH IT ROTATING COUNTERCLOCKWISE. ADJUST THE LITTLE PLATE (TENSION) SO IT WILL EXTEND FROM YOUR FINGER BY ONE INCH WHEN BOBBING.
4. INSERT EVERYTHING.
5 THREAD THE TOP CORRECTLY AND ADJUST THE TENSION TO FOUR. IT MAY NEED ADJUSTED SLIGHTLY.
6. TEST THE TOP TENSION BY PUTTING YOUR FOOT DOWN. WHEN THE THREAD IS PULLED ON IT SHOULD HAVE RESISTANCE. PUT YOUR FOOT UP NOW. PULL ON YOUR THREAD AGAIN. THIS TIME THE TENSION SHOULD BE SLIGHT. IF IT ISN'T THEN YOU HAVE A SNAG BETWEEN THE NEEDLE AND THE NEEDLE SPOOL. MOST LIKELY, IT WOULD BE THE THREAD WASN'T MADE TO GO BETWEEN THE TWO SAUCER PLATES OF THE TENSION CONTROL.
7 INSURE THE BOBBIN HAS TWO INCHES OF THREAD HANGING. TURN THE HAND WHEEL TOWARDS YOU (TOP OF WHEEL) WHILE HOLDING THE NEEDLE THREAD TOWARDS THE BACK OF THE MACHINE. YOU SHOULD NOTICE THE BOBBIN THREAD TO COME ABOVE THE NEEDLE PLATE. WHEN THIS HAPPENS PULL IT TO THE BACK.
8 SLOWLY, TRY TO SEW SOME RAGS TO TEST THE MACHINE. IT IS LIKELY THE TENSION WAS NOT SET PROPERLY.
9. ANOTHER THING THAT WILL CREATE A PROBLEM IS WHEN THE HOLDERS OF THE BOBBIN CARRIER ARE HOLDING DOWNWARDS TOO HARD. THREAD HAS GOT TO SLIDE UNDERNEATH OF THE HOLDERS. YOU MAY HAVE TO ADJUST THESE.
10. ALWAYS MAKE IT A HABIT TO CHANGE THE NEEDLE AS A FIRST CHOICE FOR TROUBLESHOOTING.

While I know it can only go in 1 of two ways, maybe the problem isn't in how the bobbin is in, maybe it is the route of the thread.
My assumption is you don't seem to pick up the bobbin thread and it bunches. If so the carefully follow the thread to needle you may have by passed the place you put that thread so it maintains tension.
Check that carefully, sometimes that gets overlooked.

Bobbin thread breaks
1. There is a right way & wrong way to insert the bobbin in the case --when you insert the bobbin in if it doesn't stitch correctly turn the bobbin over see if that helps or bobbin case is not inserted correctly
2. Lint has collected in the bobbin holder
3. The bobbin is damaged & doesn't turn smoothly
4. Lower thread tension is two tight

here's a free manual
http://www.bernina.com/getdoc/48e8d09f-98f7-40ab-8663-a23663df4fae/Bernette_715_730_740E_manual

check the upper thread tension
make sure all the thread guides all used- & that the thread is not hanging up some where on the machine
check the top thread tension dial to see if there is any thread caught in it
change the top & bobbin thread- make sure the thread is not knoty or old

place spool of thread in , route it towards your bobbin holder, take thread and put through one hole about 4 inches hanging ontop. press it in and slide it to the right where you hear a click, hit the peddle, should be winding, it will automatically stop on its own when full. click it to the left when done, snip it and then drop that into your bobbin holder , cover your plate, thread your upper thread and presto you are now sewing with your threaded bobbin, always remember these steps. #1 material choice #2 thread weight #3 needle size. you got to match those 3 for beautiful perfect stitches.

If you lift up the thread holder, the bobbin holder is inside. Loosen the hand guide to stop the needle from going up and down. Pull the thread around the top thread guide and back into the bobbin holder. Thread the bobbin and move the bobbin over so the guage is "inside" the bobbin. As for the manual, the Singer Merritt 2404 is applicable to the 2430 if that helps.

If the bobbin case/holder is trying to lift up and the thread is staying partly around it, your bobbin case may have turned part way.
Remove the needle plate and the bobbin case. The outer edges of the bobbin case should be very smooth. I suspect you may see a needle hole in the flat area that was under the foot. If so, you will probably wish to take it to your dealer and show them. They may smooth it or recommend replacing it. When the bobbin case is placed back into the hook cup that it sets in, the retainer at the 11 o'clock position on it should only allow the case to lift up about 0.8 to 1mm at the most. If it will lift up more than this, press down firmly on the retainer and move it closer to the bobbin case. You must have enough room for the thread to pass between the retainer and the bobbin case, but too much clearance will allow the bobbin case to turn in the machine and it will be damaged by the needle.