I agree, must be some sort of air purge issue. I've had good success using the suction from my pool vac disconnected from the navigator and held against the wall to clear the line.
But if you get too aggressive you can add air by running the tank empty. The only other thing I can think of is you have an issue with the pipe doing a whoop de do in the ground (I know super techy). Being that it changes level too many times in the ground and you are hydro locking the liquid in the pipe and the tank doesn't have enough volume to force the flow.