I know this might sound a little too simple and I am not trying to insult your intelligence, but have you checked the PTO levers AND lights?
After the dealership dropped off my tractor (which included starting it to get it off the truck) and left, I tried to start it and nothing. Got the owners manual out, checked all the switches and levers and still nothing. Finally, I noticed that the rear PTO light was on even though the PTO lever was in the off position. When I bumped the lever back the light went off.
I haven't taken the time to adjust the switch, so the PTO light blinks on and off most of the time, but it will not start if that light is on.