Losing prime

[zipper8096]

I have a Pentair sand filter with a 2.5hp pool pump, I know its big but filter is located 30 ft from the pool. I bought the pump after I noticed problems with the 1hp pump that I had on it and thought that the problem was that there was not enough power to suck water that far. What it was doing is it would prime itself and then lose the prime, then re-prime itself and lose it again, so I bought the new pump. However it is still doing the same thing. I have checked all of my plumbing, I see no water leaking or air being sucked in through the fittings (smoke tested). Where else could I be sucking air? That is the only thing that I can think of that would be causing this problem.