sounds like something is blocking it if it won't lift any higher.
let it back down and back the other lift out of the way and try it again.
check the chains and make sure they are not kinked up and on their rollers correctly first.
if it worked before you did that then you have done something to cause that issue from the sounds of it.
Just be careful and check it carefully before you do anything, if something is hung up it may come loose and the carriage or mast may drop suddenly, that would be bad for you if you happen to have your hands somewhere they shouldn't be.

The last thing I lifted yesterday was my buddys forklift. So whats that 6000 lbs? I lifted it 8" to put it on blocks flawlessly.
So now I just lifted the mast of this P8000 with my other warehouse forklift about 4' off the ground and tried the hydraulics again. Very slow to lift, but the cylinder lifts. Only lifts to the height that the other forklift is holding the mast to. Wont lift any higher. Am I on to anything here? Wont lift the forks empty otherwise.