Omaha here's another possibility, I don't know how long your out let hose is but when you have longer runs and the hose changes elevation more than a few times you can get air locks in the high spots which increases back pressure, try to even out the hose as much as possible, if you still have problems put a short outlet on the pump to see if it's the pump or back pressure in the hose.
Just my 2 cents.
