Sounds like you're losing prime.

Hold the end of the vacuum hose in front of a return line, pushing all the air out of the hose.

Then, don't let any air back in to the hose and hook it up to the skimmer. You may still get some air in the pump basket but it will prime more easily. It's okay to let the pump run for a few minutes with air in the basket.

You should see a steadily increasing water flow (in the pump basket) until the prime is complete.