If pool pump starts losing pressure...
1. Pump has occasional difficulty priming.
2. Strainer pot window (top lid) shows a lot of churning and bubbling instead of even flow of water through it to pump.
3. Air bubbles are coming out of return jets steadily.
You have a suction side leak somewhere.
Air bubbles in pool means air leak on suction side - the front side - going into pump.
Suction (intake) or front side of pump is where water is being pulled into pump, along with air.
Air is being sucked into suction (front or "in") side of pump, usually not the discharge side (out) with air getting into system before pump propeller.
After pump propeller, water is forced out to discharge side and on to water filter.
Top 3 reasons:
1. Is water level in pool high enough?
2. Is strainer pot lid on tight? Is its O ring seated OK? Is its O ring lubricated sealed OK? Remove any debris that might be obstructing lid.
3. Check any unions (fittings) between pool return and knurled fitting going into pump. Disassemble this fitting, check its O ring, it needs to seat into union, inside the groove. Make sure flange is tight on back of nut. Pull plumbing apart and seat O ring into groove.