Dirt remains in the filter, until you remove the DE -- and the dirt trapped in it -- by backwashing or by opening the filter can cleaning it manually.

A DE filter works when the waterborne DE powder is trapped on the filter membranes or septum, and remains held their by the water flow, and resulting differential pressure across the membrane, sort of like a sponge stuck to the nozzle of a vacuum cleaner. When the pump is turned off, the DE is released and falls to the bottom of the filter, along with the captured dirt. When the pump is turned back on, the water circulation stirs up the DE -- AND the dirt -- which then swirls around the chamber until the DE is again trapped against the septum.

HOWEVER, until the septum is fully coated with DE, dirt in the water is free to flow THROUGH the septum, since most of the particles are too small to be caught by the fabric of which the septum is constructed.

BOTTOM LINE? It's probably time to manually clean your filter. Wear old clothes and shoes, and plan to take a shower after!