Belt part constraint

First, the belt part is not a candidate for a mechanism constraint.  A solid belt will not move around the rollers at each end.  You should constrain the belt by the axis at each end and offset the side of the belt from a parallel surface.  I believe the belt can be assembled to the rollers as long as assumptions is turned off and none of the constraints prevent movement of the rollers.

As far as the regeneration, without the model, I would suggest chasing down the issue or issues, by dragging the insert bar up the tree until regeneration is satisfied.  The next part in the tree contains a regeneration problem.

I use Windchill with Creo and I see that WC/Creo feels always something changed in assemblies where are used belts.
Probably there is something that need to be regenerated (CTRL+G) and, when I regenerate the top assembly, THAT thing change and Windchill gives to me the need to put it in check-out

I think that the problem could be the kind of constraint of the belt, because If I don't constraint it I don't have the warning. But If I don't constraint the belt, it doesn't "follow" its assembly when I move it, returned in its right position only afther a regeneration; but then WC feels the assemly change and ask me to put it in check-out....