Pump pressure and hayward ultra pool vac.

[fabs]

Hi folks fabs here. We have a kdney shaped inground pool
(aprox 80 000LT). We have the sta-rite cristal flo sand ilter and the sta-rite dyna-glas pump. The reading (After backwash) on the filter is 12 PSI. Is this a normal reading for this size pool?
secondly,
We have a hayward pool vac ultra. It works for just a few minutes (not well either) and then it stops moving. I don't know if maybe it's the pressure that is too low? anyt ideas? thanks
fabs