Check for binding of the material forcing it back up. Or piling up on the pads on the wheel wells. The balance links can be installed backwards, which will jam the folding, but think it will happen way before full-down. Unzip the rear curtain and lay it in first to be sure its not wadding up as well... lower slowly checking as you go to see if something is binding.
"Could" be air in the system, but a long shot. Run it up and down a few times and check level, may be over full..??