I have a John Deere riding mower 100 series model #gx155ca026001. The blades will not engage when I pull the knob next to steering wheel. I checked belt and nothing is jammed. I was mowing some thick, tall grass prior to this. I took belt pulley covers off and there was a lot of grass jammed in there. I removed the grass but it will not engage. What do you think my problem is?