A couple of things we've noticed on boats over the years experiencing that problem.
The extrusion is cut too close to the sheave and needs to be cut down so that it does not come in contact with the sheave. We've done this a few times without taking the sheave itself off by just hack sawing the corner of the luff track extrusion off.
The other issue we've encountered is from resting the top of the mast on the ground (dirt, sand) when stepping the mast and freezing up the sheeve because of the grit. In that case, take the sheave off and clean.
If either of those don't work then post some pictures for us to take a look at to help resolve.
Hobie Factory Team