Did you open a faucet and let the air you left in the lines bleed out.
I mean I keep hearing the line about "Air in the lines" (propane) when a furnace or water heater does not work after several tries (This by the way is BS. Fridge its possible but not very. but for a Furnace or water heater it's BS)
BUt air in the water lines CAN indeed compress enough to prevent the pump from priming.
Also Some systems have a 3-way valve on the pump INTAKE (one more though too) that lets the pump suck pink stuff out of a jug.
Finally.. MY system the fresh tank drain is in the line from the tank to the pump. if I leave it open it can suck air.