Reminder:
Your question made me take a look and clean my machine. I took off the throat plate and a wad of lint was on my feed dog plate. Then I took off the bottom plate and looked at the bobbin from the bottom. Found an old piece of thread still behind the bobbin unit. Wiggled it out with tweezers. Checked everyhthing else out and now it does move much smoother. This is a good thing to check regularily.