Low Flow Reading On Pressure Gauge.

Got a 14K gal in-ground SWG with a Pentair Tagelus sand filter and Purex Triton pump. To my knowledge, all equipment is original from 2001. We bought the property a year ago and everything has been working as advertised with me performing routine maintenance incl replacing the spider gasket in the valve assy on top of the sand filter about 3 weeks ago (*!@!!grr!) because the backflush port was leaking during normal pump ops. The pressure gauge on the valve assy reads 20-30psi during normal ops, with 30psi being suggested backflush range.
Before leaving for a week back on 12 July, I ensured the pressure gauge was reading in the low 20s by briefly backflushing and ensuring everything was set up for regular ops while we were gone (pump turns on automatically around 10pm, shuts off around 4am using the creepy crawly. this is normal day-to-day function).
On returning home on the 22nd I noticed a bit more rubbish on the pool floor and checked the pump function. Though the pump sounds fine the water flow, as indicated by the pressure gauge, is very low - around 8-10psi. We've checked both filter baskets (skimmer and pump) and tried a backflush but got very low flow there as well. I've since rechecked the spider gasket to make sure it's still intact and it's fine. Is this how a pump behaves when it's crapping out. It's the only thing I can imagine. If the plumbing or sand filter was clogged wouldn't the pressure gauge read higher rather than lower?