Hi Suzi,
you may want to double check the mast channel tracks where the rollers run and make sure there is no type of debris or dirt. Also check the thrust sideplay in the mast channels and see if there is any movement from side to side. If there is no movement your channels may be too tight inside thier respective tracks. I have seen this before and sometimes scraping the roller track helps, even applying some lube where you see rubbing inside the tracking area. I know a roller track doesnt require lube due to the nature of the beast, grease and rollers tend to cause the roller to slide and wear flatspots on the roller but in some cases this may be the only solution to your problem for the time being till the mast wears in some.

Is this a new machine? If so what type mast does it have?
make model of truck?

If there are any adjustable side wear plugs check those and make sure they arent too tight.