Hydraulic lift cylinder

Howdy people, I'm new to this group, so please bear with me. I've got a question on the main hydraulic lift cylinder on a JLG platform lift. The cylinder had hydraulic fluid pouring out of the top end, right where the rod pushes out. I took it out of the machine, had it professionally rebuilt, put it back in the lift, but now it won't extend or retract when operated. I had raised and propped the platform arm up so I could get the cylinder out, but it won't extend in order to get the top pin back in. I can hear it straining the motor when the control is worked, but it's not moving. Any ideas what to do would be greatly appreciated, thanks