DE filter causing flow problem for SWCG & auto prime on pump

{equipment models in sig below]
When the pool was new I had no problems with auto prime or low flow on the SWCG. After the patio around the pool was done I started have problems with the pump not priming. Thinking it was the mess from construction I backwashed the filter but, it didn't really help. Then I completely cleaned the filter and membranes but, had inconsistent results, sometimes it would prime then it wouldn't. Eventually it stopped auto priming completely. I called Hayward & they told me to set the pump to 3 minute prime instead and I should lower the max RPM setting to 1750 rpm because the PB only used 1.5" plumbing to & form the pool. (2" @ the connections to the pump & filter). But that didn't help the low flow warning to the SWCG. They said I need at least 11 GPM to keep the SWCG going. Originally I could lower the pump to speed 1 which is 1000 rpm without a problem but, now I need to keep it over 1600 rpm or it will shut down. Had a hayward tech come and check the flow sensor but, he said it was good but, he replaced it anyway just to be sure. I then tried the valve on recirculate which bypasses the filter and both the auto prime & flow sensor work fine back down to the lowest pump speed. So if the filter is clean what could cause this?

Can CYA clog a DE filter? Because I'm having the same problem as last year and realize it happened after adding a bag of CYA spread out into the pool not the skimmer. And the Hayward tech told me to add less DE after backwashing which helped last month but, then i added the CYA.