Maybe Waste, Waterbear, or someone else that knows for sure will chime in but I've seen this with Watermoms old AG pump before we changed to an IG pump on her AG pool. Almost like the silly things would vapor lock and not pump. My guess, and only an educated guess, would be the IG pumps have closer clearances and are designed to self prime and have the capability to creat enough of a vacuum to raise water at least 12" or so since they are typically located above the water level. A trick is to fill the hose with water by holding one end in front of a return before hooking it to the skimmer. You minimize the amount of air that gets in.
Al