i had a problem like what you described a few times recently...there was a tiny bit of thread caught in the lower part of the machine, between the part that holds the bobbin, and the C-shaped part that rotates around to pull the upper thread across. That probably doesn't make any sense...hold on.See the tip of the seam ripper?
That's where the thread has gotten stuck, and it completely stops the machine from moving any further. It will move back half a stitch, but not forward.
Now, to fix it, i used a combination of rotating the handwheel back and forth, pulling at the tiny thread bit with tweezers, and getting frustrated to the point of taking apart the lower half of the machine, which was what finally did it. I hope, if this is the problem, that you have better luck getting it out than i did.
i already tried that. not a bit of thread to be found. im stuck. just like my stupid sewing machine is. stuck. thanks though!