OK, that's more information.
Since it SHOULD be working and, as Al said, you know more about it than you think, I'm going to toss out more random ideas to check.

Can you vacuum to "Waste"? If so, do that if you have to and add water. It sounds like there's a lot of junk in the water.

Does it backwash OK? first run water to waste for a little, like 30 seconds, then backwash. The sight glass should go clear (with a little sand) then very cloudy, then clear. It should run clear for at least 30 seconds, then put the handle to "waste" for another 30 seconds, then you are done.


Finally, was the sand marked "filter sand"? If it doesn't specifically say "filter sand" for pool filters, don't buy it. Look for the weekly sales (in my area we have 7 different pool stores I can choose from, plus Home Depot and Lowes) and when sand is on sale for $5-$6 buy 4 new 50# bags. Empty and replace your sand, and while it's out, check the laterals for clogs and damage, fixing if necessary.

As I said, I'm tossing out ideas and that's a $20-$24 way to try to fix it.