I have a Mey diesel tractor that seems low on hp and need some help.?

My Mey has a Perkins 165 diesel and should have 60 hp. She does not smoke or use any oil. She idles up and sounds really good, but she lacks power to pull a hill. I have changed all the filters in the fuel and air system with no increase in power. I have tried additives and have checked all the fuel lines lines. Any ideas on what is going on?