If your John Deere lawn mower deck won't engage, there are a few things you can check to try to fix the problem. First, check to make sure the PTO switch is in the on position. Next, check the PTO clutch for wear or damage. If the clutch looks good, then check the PTO belt to make sure it is not damaged or excessively worn. Finally, check the deck itself for damage or wear. If the deck looks good, then the problem may be with the deck drive belt.