It could be a lot of things. Temperature is a big one, are you letting the bed heat up completely for a consistent amount of time before each print, could heat be impacting your measurements, some things measure different when heated a bit? You probably aren't printing enclosed, but my voron for example expands upwards almost an entire layer over the first hour of printing. Is there an intermittent air flow that could be impacting how hot your bed really is?
Have you verified there is no slop or backlash in your z lead screws and connections? I had decent results on my sidewinder using POM anti backlash nuts. Same for the stability of your measuring tool.
Have you checked that it's not z tilt from turning off the motors between prints?
The bed is completely heating before each print, I just about always let it preheat and do something else before I start the print
It is not enclosed, would that make that much of a difference? I'm talking like one print turns out great for the entire duration and then I go to print again and the first layer won't even stick.
Not sure what you're even asking with the other questions about slop, backlash, and z tilt. Will have to look into those
I ran into this with my e3v2 a LOT. Are you heating up the bed and nozzle before you level? You may have answered that and I missed it. Also, I'm not sure how well those silicon things work, I did the "yellow springs" and then make them all equally tight, then level by loosening. Another thing to check- is your build surface flat? Check when cold and hot. My glass bed wound up warping up near the middle ever so slightly and that would be enough to make a batch of moms spaghetti. I was able to get it working well for a while, then it somehow shit its pants into the silicone sock and mucked up the whole hotend with nice black pla+.