I've had masts stick at full lift occasionally. Usually it's on trucks that are used for short lifts over a long period of time and the only time they go to full height is when I'm testing them for PM. I think it's gummy or dirty lift cylinder tubes in the section of the cylinders that hasn't been used for years.

What I do is load the forks by raising another forklift and laying the forks over the stuck forks then lowering. the weight of the second truck's carriage is often enough to get the stuck upright moving again. Once or twice I've had to use a load to push down even harder on the stuck forks. You want the hydraulics on the stuck truck all connected and the lower lever actuated while loading the forks. You don't want excessive slack in any of the chains.

This is all very dangerous stuff and I can't recommend doing it unless you have knowledge of how uprights normally operate and are in a safe area with no unnecessary people around. You have to keep the area under the forks clear and keep hands out of the mast.

If that works, you can free up the upright by lifting a thousand pounds or so up to full lift and back down maybe a dozen times. Strangely, I haven't had seal leaks in trucks I've done this to. I sorta would have expected at least some packing damage.

Again, you and your helper need to be very alert and aware of your surroundings if you try this.