Assuming there is no safety interlock that won't allow the engine to run when the deck is lifted, and assuming it's in the down/cutting position when you engage it, and, assuming the deck is not seized-up, which would probably stall the engine, engaging the deck slowly should allow it to spin up. I've had older ride-ons that nearly stalled each time the deck was engaged, due to a build up of grass, or bad belts. Can you (of course with the engine off) rotate the blades easily? If not, you need to look for binding.