Could be the fluid level, you may have a leak. You need to pull the rear seat to gain access to the hydraulic motor, visually check the lines to see if there is excess air in them. There is a plug on the side of the motor where the fluid goes. Pull the plug, the fluid should be just below the plug opening and no more if it is low use a hand transfer pump(the easiest i've found) to fill the reservoir to the bottom of the opening. While the top is up, disconnect the cylinders from the top(1 bolt either side) and cycle the top up and down several times to bleed the excess air from the system while the plug is out. Also check for leaks while you are there. If you have to replace the motor or cylinders use this same procedure to fill and bleed the system.
