If all else fails, get out the manual and follow step by step how to thresd and not what you have alway's done. One of teh board members here realized after three years of owning her machine, that she had been threading incorrectly. It happens.
Not all bobbins fit all machines. Use the one intended for your machine even if you have to pay a little more.
I used a heavier thread in the top than in the bottom yesterday and had to tear it all out. Unless your maual say's you can do this, don't.
When is the last time you cleaned your bobbin area from the lint etc.? Even a stray piece of thread will clog it up.
Check the manual and see what your tension should be. Your tension could be totally off.