The water pump can not pump air. On my
Casita the toilet is the lowest point in the system. I hold the toilet flush open until water flows. One time I took the head off the shower and sucked on that too get water into the pump.
The way I've found too prevent this from happening is to open the bathroom sink hot and cold water faucet while filling the water tank. Then over fill the water tank. This pushes water into the water pump.