Perhaps the mender nut got put on backwards or in the wrong place?
With the hose disconnected from the barbed end(wall fitting), put the mender nut on the wall fitting with the shoulder of the nut facing away from the barb, or towards the wall of the pool. Install the hose all the way to the shoulder of the barb fitting, about 1 1/2". then just thread the mender nut onto the hose. It should tighten down and become difficult to tighten any more. If you can continue to tighten more than say an inch and a half, then the hose will no longer hold the barb and you'll have to replace the hose. If the hose is misshaped on the end, you can try cutting the misshapen portion off, 1 to 2 inches, and try again.