Inground 15k gallons pool.
Nautilus FNS 48 DE filter, with cellulose fiber

The problem, what I have observed and what I have done so far....

Pool runs every night from 8pm to 5am. The other day I walked out at midnight to find the pump running but no water was returning to the pool-reading was close to 30psi. Next day I did a back flush, 3x, and it was running ok-psi was 15. What I noticed is that after about 3 hours of operation return flow quantity starts to tapper off, pressure starts to trend up. If I let it continue running by midnight or one the return is just trickling back into the pool, next to nothing really-psi is about 25-30ish.

Pool used to run great even with a filter much dirtier than I found it this past spring. It used to take about a year for psi to build up from 10 to say 35-37ish, then I would clean the filter. Sometimes this last spring I noticed the psi climbed up faster than in previous years so I cleaned it. That is why when I opened it again this week it was still clean.

I attempted to vacuum the bottom, with selector set to waste. I hose-fill, then vacuum as I have always done annually. This time suction pressure was gone after a short while. It came back after a backwash. Did the same thing so I stopped that. I estimate it would take 4 more of this action to complete the vacuuming process. It used to be one go and finished.

I tried to put a bladder on a hose through the skimmer but it couldn't make the 90 angle turn, to go through.