Filter problems...high pressure low output

Hi have doughboy sand filter that is dealing me a fit. At first, I would get great pressure and suction, then several hours later it would decrease. Turns out sand was building up in the underdrain assembly. I took apart cleaned it, double checked the gasket where the tub meets the valve assembly. All seems ok. Still does the same thing and start to throw sand into the pool.

I got a new one, cleaned it all out, now I get a higher pressure when I turn it into filter mode and seems like something is blocked up. All the other settings seems to work.

could this be an issue with valve assembly? Is there a inner valve or diaphram that could be defective?

All lines are hooked up correctly.

Thanks

It sounds more like you've may have the wrong sand, and fine particles from your sand are clogging the laterals. Coupling bad sand with the typical undersized AG filter and oversized AG pump could produce the conditions you describe.

Did you use labeled and graded filter sand?