Need to know a bit more to provide good help, but I'll take a couple stabs at it from what you've explained.
Is the tractor new to you? Or have you had the tractor since new and this is a new problem you've never experienced?
Is it the eHydro transmission? If so try a lower gear range or ease off the "go pedal" some. Less pressure on the "go pedal" for a HST is akin to putting it in a lower gear. More pressure equals more speed but less power. Same for FEL and backdragging. If you are increasing the pressure on the "go pedal" as the load increases, you are operating it incorrectly and should lessen the pressure as loads increase. I'm not sure about hills where you are, but hills here in WV can't be climbed in high gear by any tractor I've ever operated or been near. High range is reserved for getting from point A to point B on mostly level ground with little to no load.
If you are concerned about the power output, call your dealer. Many can dyno-test the unit from the PTO shaft easily.