If the motor is running and the top stops lifting, you are either low on hydraulic fluid, or have binding between the top and bottom halves, or possibly, a leak in the system. If you're not seeing hydraulic fluid on the ground, then the lowering valve may be partially stuck open.
This "stuck open" problem can be because you have opened the manual lowering valve partially (if you have one) or, there can be debris stuck in the valve. You can clear debris by raising and lowering the top repeatedly to flush the valve.
If the manual valve is open, close it. If you're low on fluid, add some. If there's binding, you'll have to find it, which can be hard to do. Sometimes, things fall down between the two halves.
- Jack