I had similar problems with my taz6. Using a dial indicator mounted as a printhead, I found more than a 1mm difference between the back right and front left corners (after calibration). This was using the cura (not cura2) firmware. With the cura2 beta firmware it was sometimes better and sometimes worse because this firmware was only using a 3 point calibration to create a plane whereas the cura firmware was using all 4 calibration points and averaging the planes that are generated from those 4 points.
After going back and forth with support, I was sent a new aluminum plate. Using the cura2 firmware, this made things a lot better but not great. I could print a full bed calibration print using PLA if I applied glue stick to the bed (but not without the glue stick) because of the lowest corner.
Updating to the latest firmware (which may or may not have been released yet, I was building it from source), it is now generally okay. The newest firmware uses a different calibration algorithm that takes all 4 points into consideration and produces a better calibration (for me). With this version and the new plate I successfully printed a full bed calibration print without a glue stick. While it succeeded, it just barely succeeded having some minor issues in the corners (too close / too far from the bed). I doubt I would be able to get 3 calibration prints in a row to succeed but it’s close enough that I believe I could print a very large print using a glue stick to buffer the slightly too low points and it would work fine.
My take-aways for someone finding this topic are:
- don’t rule out the plate being warped, but I’d suggest trying to verify that (I used a certified straight edge to check it).
- older firmware may be better than more recent firmware but worse than the very latest firmware
and, frankly, just calibrating off the 4 corners has some limits in how well you can automatically level the bed. After trying to understand what was wrong with my printer I understand that it is unlikely to ever be 100.0% perfect. Calibration from the corners cannot capture a more complex problem like concavity or convexity in the build plate or non-planar deformations in the corners.