Priming the water pump

Hello all- We have a 6 year old travel trailer and over the past few years we have had an annoying issue with the water pump that I am getting really frustrated with now. If the unit sits for a period of time with an empty tank, or if we camp with hookups in between outings where we use the tank, I can't seem to get the pump to prime on its own the next time we try to use it. It will turn on with a quiet hum and never do anything. When this happened in the past I used to be able to put my finger or something else in the outside tank vent hole for a minute and then release and that would work but I have not been able to make it work. The only thing I can do, and have been doing, is to disconnect the inlet hose from the pump, put a funnel directly into the pump and pour water in and reconnect the hose. That seems to cause a prime that then works as long as we keep the tank filled and keep using it.

Any idea why the pump no longer primes on its own the way it should? Thanks!