Did you compare the new belt against the old as regards length and width ? Except for old old dryers usually Maytags which used quite a long belt with 5 ribs the belts I see for sale are usually specified as being for a 27 " or a 29 " wide dryer. Using a 29 " wide dryer belt on a 27 " wide unit will not have much tension and it would slip. Conversely if you tried to put a belt in place that was the type intended for the 27 " wide unit it would be difficult to put on if you could get it on at all. Do the number of grooves on the pulley match the grooves on the belt ? A skinny/narrow width belt will work on a 5 groove pulley but a 5 groove belt will not fit properly on a narrow pulley. Another thing to look at is the spring tension for the idler. Any problem with the alignment ?

Might be a good idea to see how difficult it is to turn the drum starting from no rotation. Go to an appliance store and check a new dryer to see how much force it takes to turn a new one as compared to yours. Again it might still be a good idea to see if there is any extra drag on the drum regardless of the belt situation.