I have had problems with some of my machines over the years. And, I find that if you take everything off of the machine and start right from scratch - read the book/instructions on how to thread it and how to put the bobbin in VERY carefully and do each step as you read it. This has helped me find the step that I somehow missed, or just thought I know what I'm doing - don't need to look in the book!
In fact, I have 1 machine that the bobbin thread has to come from the back and off to the left of the bobbin, while I have another machine where it has to come from the exact opposite side and off of the front of the bobbin. IF I do it backwards - nothing but problems!!!
So, just pretend that you have just gotten the machine and follow each step AS YOU READ IT.
Hope this helps you.