Constant buildup under the shower screen

Having a weird issue with my Gaggia Classic. Over the last few months, I get build up that blocks the shower screen after just a couple of shots.

It clears up if I clean the thing. I take apart the group head, soak everything in cafiza and scrub it down, backflush (again with cafiza), and then it's good as new. But within a couple of days, it's blocked again. I've gone through this three or four times down.

This is a new problem. I used my machine for well over a year and never dealt with it. I don't think anything has changed about my set up or grind.

Any idea what's going wrong? Am I over or under tightening the shower screen or something?