More likely though the overflow or malt pipe handle holes. I use plugs in the handle holes, are you sure the grist is not going through these? Seems unlikely the grist would get past the screen. I have not had a pump blockage with the same screen set up as you.I have the KegLand BrewZilla 35l and use it with with malt pipe plus the fine stainless steal mesh as well. While mashing I run the pump to maintain temperature in the mash bed. Most times the pump blocks with some grain getting through I'm guessing between the malt pipe and the bottom screen in the malt pipe ? I do stir a couple of times while mashing. Is this something anyone else is experiencing? Something I'm doing wrong?