Feed dogs don't want to feed.

Question

I have a Bernina 180, a lemon I might add, and my newest problem is the feed dogs don't push thin fabrics through without puckering them and hanging up completely on seams.
By thin fabric I mean rayon or cotton. I have adjusted the pressure on the presser foot to have more contact with the feed dogs but I still have to pull the fabric from behind to prevent it from bunching behind the pressure foot.
I had it timed and tuned only a year ago and do not use it for weeks at a time. I am a seasonal sewer .Do you have any suggestions besides another tune up.