When I brew espresso, I often get "bald" spots where no espresso is running through the some parts of the filter basket. Or in other words, I can see the filter basket screen whilst brewing coffee. I reckon this ins't the right terminology so here's a picture of what I mean:
My setup is a Lelit Bianca with a VST basket and one of these "nanotech" shower screens. I grind with a Baratza Sette 270wi but I single dose the coffee (i.e. I'm not using the built-in weighing function of that grinder). The coffee is fresh (~7 days from roasting) specialty coffee, light to medium roast. The pump pressure is set to ~9 bars and for the sake of fixing this issue I'm not using the pressure profiling functionality of the Bianca. It's full stream for 5 secs (pre-infusion), then off for 4 secs and then on again to brew the coffee.
I put 18 grams of coffee in and I'm aiming for ~38g of coffee out in 25 seconds after the first drop of espresso lands in the cup. With the pre-infusion that means the Bianca's shot timer will show something like 34 seconds (9 sec of pre-infusion -- 5 on, 4 off) + 25 sec for the actual brew.
I carefully prepare my freshly ground coffee with some WDT and a tamp. To me, my puck prep is looking okay because can see that the first bits of coffee show up more or less evenly all across the basket. Here's a picture of what it usually looks like on the first drop
After the first drops fall into the cup everything starts to look nice for maybe 5 seconds but then the "bald spots" start appearing. Here's a picture of what I think is looking nicely
It seems to depend a little bit on the coffee I use. Sometimes it happens more and with other coffees it happens a bit less. But there's always the tendency (or with some coffees every single shot) for the bald spots to appear pretty quickly.
What am I doing wrong here?
Some things that don't seem to matter much here are grinding either finer or coarser: As you'd expect that greatly affects how fast the espresso is running through but the bald spots don't disappear. Also reducing the pressure or trying different kinds of flow profiles also don't seem to really affect it.
Oh, and the wet puck usually looks okay, there are no obvious signs of bad channeling at least to me. Here some pictures of the spent pucks
I can't see anything obviously wrong here but I might be missing something. In case it helps, here's a video of a shot affected by this issue.