Pat have you pulled the base off and checked the legs, bushings, and cavities to see if the is anything preventing it? Try pulling the springs out and see if it will fully retract without them. About all I can suggest right now is to try looking for something that isn't right and try changing things as possible to see if something makes a difference. That might point you in the right direction.
I might also try lubing the legs with dry graphite just to see if that makes a difference. I'm also wondering if maybe there's enough wear to allow the machine to cock a bit with pressure put on it and causing it to bind.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.