not sure what you mean.
The belt should be loose with the blades turned on and go tight when the PTO lever pulls the cable to move the tensioning pulley .
IF thta does not happen they you either have the wrong belt or the belt is routed wrong.
How old is the deck and what are the pulleys like ?I have a d110 with the same issue. The belt will not tighten as it should. New belt, routed correctly and I am dumbfounded.