I had a similar problem creep up last year. I thought I had a tear in the fabric of the D.E. grids so I replaced them and I still had problems clearing up my pool this year. I realized while trying to open the pool that I could not keep prime in my pump for any appreciable time. When I turned off the pump, I would hear gurgling noises coming from the multi-port valve. I did some digging and found that there is a gasket (spider gasket as it is called) in the valve that was allowing dirty / d.e. laden water to bypass the filter and go directly into the pool. After replacing the gasket, I can keep prime, but best of all my pool cleared up super quick.

If you have a multiport valve, I would, at a minimum, inspect and lube the spider gasket with aqua lube. The gaskets are cheap, I think I paid 20 bucks or so, and easy to install.

Good luck.