I had the same problem and the answer is that you have a small leak somewhere in your
lines. Tighten all your hose lamps and listen for a leak. If unsuccessful, look at the
pump carefully and bypass filters and hot water tank until you isolate the problem. The system should be self priming.
We empty the tanks after each visit and refill, simply letting the Par
pumps self prime. After a season finding a plethora of leaks, we finally had a tight system. The final leak turned out to be a poorly installed filter.