I'm not clear: is the pump priming a NEW problem, or one you had since you had the pool?

You can test the 'rocks'. Both borax and stabilizer can 'clump' up or solidify, but borax will dissolve fairly readily in hot water. Stabilizer will not. Dissolved borax will make the pH HIGH, if tested with phenol red. Stabilizer will tend to make it lower.

If you find it's borax or stabilizer, I'd try to hook up a sump pump, placed in the pool, to a hose, and hook the hose to the pool pipe. If you can force even a small amount of water continuously past the borax or stabilizer, it will dissolve eventually. You could use the hose connected to the house piping, but you'd need to be very careful not to overpressure your piping, and damage it.