I have quite some difficulty with first layers on my Voron 2.4, so let’s take a foray into investigating the build platform and related components and configurations, shall we?
My generation of Voron 2.4, configured as recommended at the time, finds mechanical position Z=0 (nozzle exactly touching build surface) by pushing the tip of the nozzle down onto a (heat-resistant) stainless-steel pin [the lonely white dot below the toolhead], triggering the Zmin limit switch, and then applying the recorded offset between the trigger position and the build surface.
This is brilliant in that it automatically compensates for different nozzle lengths and even compensates for nozzle thermal expansion, and the evil opposite of brilliant in that you have to make a new Klipper printer.cfg file for each different build surface and they tell you in the Klipper forums that you’re doing it wrong if you ask for a better way to maintain different offsets for different surfaces. And then the Voron team invents the Voron Tap, which is a better way but which I haven’t implemented yet on this printer because reasons.
Oh dear me, I seem to have meandered a bit. I think I was about to say that the first step of figuring out inconsistent first-layer behavior would be to check the repeatability of the Z limit switch. Yes, that was it.