Poor Suction from Skimmer

I've searched through this site but haven't found anything that is specific to this problem. Pool is open, no bubbles appearing in pool that indicates air is being sucked through the lines But...the suction at the skimmer is so low that I don't even try to vacuum through this connection. (Thank goodness I have both a polaris and a robot vac!) We've checked all the plugs at the pump and pump skimmer, at the filter, and at the polaris. All seem to be tight and not leaking. Any ideas as to what is causing the poor suction? This is an inground gunnite 30k gallon pool.

What is your current filter pressure? What is your typical clean filter pressure? Sand filter, cartridge or DE? Have you backwashed any?

What kind of filter and model and the pump size? Also, if you have a 'bypass' or 'recirculate' position on the filter valve try that to bypass the filter and see if the flow improves. If it does then the filter is plugged.
Al

It's a sand filter, Tagelus, which we changed the sand in about 2 years ago. The normal pressure (in previous years) was consistent around 15, and that's what it's showing now (I just bought a new pressure valve to be sure).

I turned the filter to recirculate and yes indeed it does seems to be moving the water faster and there's greater pressure in the skimmer basket (not a lot, but definitely a discernable difference). One funny thing (maybe) is that on recirculate the pressure reads 12 instead of 15. And another thing: the pressure went up to 25 just a hour ago, after only 2 hours of filtering. I haven't seen pressure that high in several years. I backwashed and it went back to 15.

The pump is a Pac Fab at 1hp.

So, does this sound like a clogged filter? Could it be a leaking spider valve? Or the sight glass maybe leaking? I'm totally befuddled.

Thanks for your help :-)

Definitely sounds like the sand is doing its job and catching something that's plugging it. The drop in pressure in recirculate mode is normal...no resistance from the sand media. The 25 psi is probably almost the blocked pressure number. Seems like plugged fiilters are 'in' this season as there are quite a few threads about the problems. One suggestion is to use skimmer socks to trap crud before it hits the filter. A pain changing and washing them so often but beats using precious water to backwash. Give them a try until the crud is cleaned up.
Al