Are you saying that in the following scenario that you cannot lift the front end of your tractor?
Step one: raise loader until bucket clears the ground, then point bucket edge straight down. Flat lower portion of bucket is now vertical.
Step two: Make sure your feet are off of the brakes and parking brake not set, so the tractor can rotate around the rear axle.
Step three: push control stick forward slowly, the bucket edge should now touch the ground, then continue holding the joystick to continue the downward travel of the bucket. the front wheels of the tractor should come off of the ground.
If wheels do not come off of the ground, then your PRV, pressure relief valve must be set too low, or pump has bypass, etc..
You mention you can lift a full load of material to full height.. What Materieal? Dry leaves and ping pong balls weight somewhat less than say wet sand or iron bars
A pressure test is the only way your are going to know what is wrong. You should be able to lift the front end of your tractor easily. It is a p*** poor test of lift ability. It is very easy to lift your front end, vs lifting a bucket of heavy materiel. So if you say you can lift a bucket of material, but cannot lift your front end off the ground, then that is why we are all puzzled. It doesn't make sense.