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.