Most dryers have an spring loaded idler pulley ( similiar to a car)
that puts tension on the belt. The pulley is also connected to a
switch in the event the belt breaks or comes off, the dryer will shut
off, and it will not turn on until the problem is fixed.
Here is a website that lists the belt routing diagram for most dryers,
make sure that you routed it correctly.
Otherwise soundes like a broken idler pulley.