Inside the cylinder should be a relief valve, it is closed when cylinder is all the way down. As you trim up to max it unloads pressure not letting it trim up and more. Sitting still it dosnt take as much pressure to tilt motor up so it stays closed. Sounds like your relief/ trim valve is sticking slightly open and leaking not letting it trim up all the way. Need to check fluid and make sure it's full and no water in it. One thing you can try, it has a bypass screw somewhere on the trim/tilt set up. Find that screw and open it up, with young of trailer lowered all the way grab foot of motor and lift all the way up and dop a few times. That may help unstick valve. Then screw it closed and trim up and down several times.