Pump chamber won't fill completely, cavitating all week, no more full pull

[fathom39]

In-ground 24000 gallon pool
Spa with spillover
Water level 1 inch low but still plenty for skimmer ports
Pump behavior the same whether pulling from all pool or all spa
I cleaned and re-lubed the O-ring plus cleaned the cover plate and mating surface on pump housing

Unusual in the last week: I rotated (electric servo) the three-way valve right before the pump without first turning off the filter. But, this was pool <--> spa and I don't think it passes through a closed position.

Also unusual, opening the cleaning port valve (right before the three-way) to suction doesn't change the pump prime.

I don't think it's an air leak.

Can it be anything other than a blockage?
Do pumps go bad (not the motor)?
Since pump behavior is the same for each source would the blockage be in the three-way valve?
The three-way is glued in: can it be disassembled in place?
What about swapping the pump with the identical one right next to it that is used for the spa jets?

 

[mas956]

If you think the water level is 1" lower than whatever "normal" is, I'd fill to that level and see what happens. That's how my pump acts when water is low. Start with the easy solutions first.

 

[fathom39]

The problem was a clogged impeller. I cleared that then swapped the pumps so the filter pump which ran every day for ten years is now the spa jets pump which runs maybe ten days a year. I should be good for a while now.