I used to work for tk in construction and we had the same problem with a jack like that that was brand new from the factory. It came missing a type of check valve that divides the pressure to make each jack move at the same time. That's been almost 10 years ago so I don't remember what it looked like. I just remember we had to chain fall both plungers out to get to it. I'm sure that same valve is what's causing your issue.
