random mesh-bed calibration failure
I assembled my MK2S kit and have had very high quality prints. However, recently I often experience random mesh-bed (pre-print) calibration failures. It goes as follows: at two probing points (4 and 9, right-center and back) the nozzle would push down at the bed, bending it slightly, and an error is displayed in the sprit of "cannot detect sensor, check for debris on nozzle, be ready to reset the printer". Initially I did notice there is debris on the nozzle which effectively drops it slightly and cause calibration to fail. But recently more and more often the nozzle is perfectly clean, and after a reset the calibration goes through, sometimes it takes a few resets or even a z-calibration. It's getting more frequent and I fear maybe some sensor points are beginning to fail--which scares me. I also notice even when calibration goes through, the calibrated height is not consistent; sometimes a -40nm z-adjust is needed to get a good first layer, sometimes no z-adjust is needed at all. Do you guys know what is happening? In the meantime, I'll do a full xyz calibration to see if the problem goes away.