I am assuming that you removed the piston & nut from the unit and checked it's operation.

Did it used to pump out OK or is this a new unit for you?

How much water does it leave in the unit?
It is normal to have a quarter inch or so left in the unit. These pumps do not like running when dry as they can burn up the pump seals.

Is the drain hose looped up at least 10 to 12 inches higher than where it enters the house plumbing.

If using an air gap is it clean internally ands allowing air in.

You could try disconnecting the drain hose at the house plumbing and draining it into a pail. Just be sure to securely hold the hose in the pail and put a towel over the pail so the water does not splash all over. If it drains OK the there may be a house pluming issue and the unit is siphoning water back out of the drain.