Originally Posted by thepolyparrot

Birds nests on the back almost always means that there isn't enough tension on the needle thread.

First thing to check - are you lowering the presser foot? (I don't even want to guess how many times I've forgotten to do this - that hopping foot looks like it's lowered sometimes even when it isn't. If the presser foot isn't down, the upper tensioner is not engaged.)

Second thing to check - when you re-thread the machine (by the manual) make sure that the presser foot is UP. If it's down, the tension disks are pressed together, keeping the thread from seating between them correctly. This will often resolve itself, but sometimes, it won't.