Hi and welcome to the forum!

Your Navigator is a suction side cleaner and I agree with Jan that you have an air leak or similar in the hose that allows it to drain when the pump is off.

What I'd do is to disconnect the hose when firing up the pump in the morning and then reprime the hose before reattaching it to the skimmer.

This shouldn't take long and, when repriming the hose, you may well see where the air is getting in