If you notice air bubbles in the pump while it is running or the pump can’t catch a prime, first check the lid gasket. It should not have any cracks, be flexible, and have some lubricant on it. While a failed gasket is a simple fix, many overlook this.
A good analogy would be to unscrew the nozzle from a garden hose and remove the rubber gasket. Screw the nozzle back onto the hose and turn the water on. You will quickly find out that the gasket is needed to keep the water from leaking out. There is no difference in the pump lid gasket, as it is required to keep the air from being sucked into the pump.
If you don’t have a spare gasket, sometimes you can diagnose the problem with a water hose. Without a nozzle on the end of a hose, allow the straight stream of water to be directed between the lid and the pump and work your way around the diameter of the lid. This will displace the air. If the pump pulls a prime or the bubbles in the pump dissipate, you have diagnosed the problem as a failed pump lid gasket. There are many other reasons why a pool pump may not be priming, another one being a clogged pump impeller.