R22 Belts
We experienced this after every rebuild that we did on R22's and R44's. We would leave them engaged overnight giving the belts time to stretch. On some aircraft it took a few nights to get the belts stretched properly.
As an aside, we found that a clutch engagement time of between 48 seconds and one minute meant that the belts were set correctly. The manual allows for a maximum of 90 seconds if I remember correctly.