Typically, nest on the bottom means the upper thread is the problem. I had my long arm doing what your machine is doing and finally, found out the thread on the top was catching on the notch that you use to keep the thread from unwinding after using it. It would sew okay a little bit, then mess up royally. I started watching the thread all the way through and sew it when it happened. I repositioned the thread spool and no more problems.
Another time it happened, it was a tiny bit of thread in the thread path up behind where I wasn't supposed to take my machine apart. The tech found it for $125. I know how to take that piece off now.
Another time, I tried using an L bobbin when I was supposed to be using a Class 15. The top thread kept catching on some metal surrounding the bobbin case. This time it was top and bottom. Changed the bobbin and everything was fine. I have marked several bobbins that are Class 15 to use for comparison. Now I wonder why the bobbins don't come with a notation as to what number/type bobbin they are? Now if I start having problems, I check bobbin height first. My L bobbins are about 1/32nd of an inch shorter than the Class 15's. Not much, but enough to cause problems. I am actually in the process of checking all my bobbin heights and moving the L's into a safe place. The thread on them is a very tiny polyester that I will use as a top thread to stitch around applique.
Another time, I didn't have the thread seated properly in the upper tension discs. It looked okay, but was just sitting in the discs....someone else had threaded the machine. They didn't tell me it was messing up on them. Re-threading stopped that problem.
With the long arm, I had some bobbins, that weren't wound properly. From the mess under the bottom, I thought it was my upper thread...nope. I looked at the bobbins carefully and realized they weren't wound evenly. I pulled off half the bobbin thread and tried again. Worked perfectly.
If somehow or other, your machine has jumped time, it will do the same thing. It's more likely to break needles and such, but if only out a tiny bit, it can make bird's nests. Now, that requires a professional for me.
Good luck!
Last edited by Barb in Louisiana; 12-28-2014 at 02:49 PM.