Good suggestion on the keyway......
In either event, relief or keyway, it's not making enough pressure.
If there is something stuck on the relief valve seat, you could try backing out the relief adjustment some (count specific number of turns) then run the system ....to clear potential blockage holding relief open, then return to original setting. Check with gauge (deadheaded) before using it much to make sure it isn't set too high. These loaders only run up to about 1500 psi on their lighter duty cylinders, so don't over do it.