Pump only primes on backwash

[MirGSS]

Ok so we seem to have fixed the groundhog hole in our pipe, but now when we turn the pump on, it won't prime.

Other info:
1. Pump primes when filter is set to backwash.
2. Water level is high enough.
3. I changed the pump basket o-ring.
4. I closed both of the skimmer valves, one at a time, and there is no change.
5. I checked for clogs before the pump filter and after the skimmer lines join back up.
6. No clogs in skimmer baskets or pump filter basket.
7. Weirs are fine; just getting very little suction (even with only one skimmer line open) when on filter mode.

I think that's everything. Like I said, it WILL prime on backwash mode. I have not tried other modes yet.

ETA: recirculate does the same as filter. Rinse *works same as backwash. 0 pressure on filter when in filter mode even though I can hear water running into the filter. Impeller and impeller o ring appear fine - no clogs or cracks.

 

[MirGSS]

Also we don't have any fancy things like a spa or a robot cleaner. Just 2 skimmers, chlorinator, and pump.

 

[Texas Splash]

Can you post a pic or two of the equipment pad?

 

[MirGSS]

Note that the heater on the left is non-functional and not hooked up to anything.

Just tried replacing the spider gasket assembly (since apparently you can't buy just the gasket anymore) and that made no difference.

 

[Texas Splash]

I only have a few ideas since you have done quite a bit already.
1 - When you replaced the pump pot O-ring, did you lube it?
2 - You just replaced the entire key diverter assembly correct? Were you having this prime issue before that work, or after?
3 - I'm wondering if prime is struggling on filter as if there is more "pull" suction required than on any other setting. Maybe a stretch, but a thought. Have you checked carefully all those connections, hoses, clamps, etc to ensure there are no other suction side air leaks getting in?