Assuming we are talking about your L3400, another thing to check is the hydrualic suction hose. Early ones were a problem and prone to cracking. This would be about a 2" rubber hose just forward of your right foot when sitting on the tractor. If it is cracked, it could be sucking air.
Other than that and what has already been said, power is a function of pressure. And if it isnt an obvious problem, you need to get a hydraulic pressure guage and a quick coupler to test the loader ports.
Put a hydraulic pressure guage on it and see if the relief is set too low. Kubota is frequently conservative when setting relief pressure. If you do a search, you should find plenty of reading on this.