Prusa Research

Hello folks! Build a MK4 4 months ago and was very happy with it so far. Now I have bed adhesion problems (see pic attached). 3 weeks ago I had similar problems and after a chat with prusa and resetting the calibration / cleaning the build plate again it worked fine. Now the problem is back despite a clean build plate . After resetting the calibration I got one nice print and then the same problem again. Cold pull of the nozzle resulted in a clean tip and no better results, so I do not think there is any clogging. Also the plate and the should be alright, as I got good results inbetween. Any ideas / hints where my problem is? Link leads to image of the problem. Could not upload the file for some reason...