When I started my pool for the season, I wasn't getting any flow to the pump unless I turned off the pull from the skimmer and pulled from the spa. I figured that maybe something was clogging the pipe from the skimmer, and once I started slamming the pool, whatever was there would die and get sicked into the pump basket. Once I start pulling in water from the spa, I can close off the spa drain and I start pulling from the skimmer. However, when the pool shuts off, and then starts again, with the spa suction turned off, I get nothing or just a trickle of water coming in. I can open up the spa suction, everything is fine, then turn off the spa suction once it is going, and it's fine. I don't see a lot of air in the pump lid, it's kind of like a bubble in the top of the dome, but that's it. When the pump starts there is air bubble coming out of the return jets initially, but it goes away.
I don't know if I am getting any suction from the main drain. I don't have a way to shut it off, but I can try shutting off both the skimmer and the spa and see what happens.