I've learned that it is important to RETHREAD my machine EVERY time I sit down to sew. That means even if I just leave the room for a minute. It saves time in the long run. It's amazing how many gremlins are just waiting for us to turn our backs.
I learned about the re-threading from my Pfaff dealer -AFTER an expensive repair. She told me it could have been avoided if I just did that one simple thing. You may NEVER have the problem I did, but why take a chance.
And always check to be sure you presser foot is DOWN before you start to sew. That rat's nest of thread on the under side of your sewing project is just WAITING to happen ~ ~ ~