Turn your mower off and check how the blades turn underneath the deck. Any obstruction, damaged pulley shaft, or seized pulley can be the cause of that. Check your belt and make sure it is properly routed and that it does not get hang up at at of the belt guides. Remove the pulley covers from the top of the deck, and check to make sure that he belt and pulleys are properly functioning. I also think that there is a possibility that your mower may have a safety interlock system that prevents the mower from engaging when there is no operator. So check on the safety switches, I believe it to be on the seat. as well as on the parking brake. This of course is only valid if your engine is properly running, that the idle is high enough and that the engine still has enough torque to deal with the mower deck. Good Luck and I hope this helps.