While waiting on the new drain plug/o-rings, take your time reviewing the following (some of which you have done I know) which can all result in some water flow problems at the pump:
- Water level in pool adequate?; should be at least halfway up the skimmer opening
- Skimmer weir door operating properly; not stuck shut blocking water flow
- Filter carts clean and unobstructed; restricted flow can cause back-pressure and the pump basket water can drop
- Any/all valves/fitting above ground in front of the pump (suction) have been inspected for leaks. You can try the garden hose method, food coloring, shaving cream, smoke, or even saran wrap at each joint. Sometimes 3-way valves let air in due to a faulty o-ring at the stem, or even a bad stem/seal itself.
- Make sure to rule-out a partial clog between the skimmer and pump. If needed, push water "backwards" from the pump intake back to the skimmer with a garden hose (wrap with a rag to help seal) to see if water comes back okay or if anything dislodges.
- All valves fully open
- With power OFF, reach into the pump basket and feel for the fan-like impeller; check to see if it moves freely and nothing got tangles in it.
Maybe one of those things will help.