that was quite short but perfectly correct..
if the driver isn't engaging for some reason.. you need to find the reason..
simplest way is open it up and have a look.
(could be a roller slipped out it's guide sometimes just tilting and preventing the driver from engaging.. could be just dirt gunk...
So as long as your belt seems to be in good condition (and no a std V belt will NOT work properly anyways.. no matter how often you read otherwise in the past few days.. just saying)
It'll be the mechanical parts inside... thus as Joe said: open it up, peek inside!
(well three rollers, three pins and three springs... plus three guides for the rollers not too fancy

)
While it's open anyways.. take a can of dry lube.. clean the inside as good as you can.. then just a little more... and when finished spray the INSIDE (and nothing but) with dry lube
'sid