Is there an aftermarket cleat for the main sheet? The guide on mine is so close to the cleat that the sheet won't pull through.
The short answer is yes. Com-Pac doesn't make this stuff, and installs gear from one of the marine hardware manufacturers. Look for a maker's mark on your tackle. I know Harken and Ronstan, for example, usually have various line guides available for the same base or cleat, that can easily be swapped out. Someone may have replaced your mainsheet with a thicker line, typically for ease of handling. There may be extra friction in the blocks as well.
I see from an earlier post that you have a CP 16. I'd go to that forum and search for "mainsheet" and you'll get lots of hits for issues other 16 owners have had, and what they've done.
+1 on Bruce's suggestion that the line may have been changed. On our Sun Cat we changed out the ever-so-slightly too large main sheet and our problems were solved.