You may have air in the system. Cycle the ram a few times then top up the oil with the ram retracted.

Is there a pressure relief valve on this thing? Relief stuck open.
I am not familiar with this splitter but I would look on the control valve for a relief valve. Sounds like you have oil flow to move the ram without a load but not creating pressure.
Remember you can have flow, (think open garden hose)
but pressure is only created with a restriction. (Same garden hose but with a nozzle on the end.)