Sounds likely to be an air leak but you can rule out amal control and pipework faults easily by just pulling the pipe off the actuator altogether. That should see you running 40psi+
the actuator seems to move at the same pressure regardless of rod length.
Out of interest, is it quite easy to move? a -34 is a real bastard to move, its very very strong indeed and takes some moving, often needing bodyweight against a large screwdriver on the shaft nut if you dont have the luxury of a pressure rig like we use.