I have a Prusa i3 with a J head .4 mm tip 3mm filament . I have been printing happily for months. Until last night when the metal part of the hotend was pushed out of the threaded part. I cleaned it out and threaded it back on. This happened three times today. I double checked to make sure the calibration wasn't off on the extruder motor and all of the axis, they were all fine. I'm using pla and slicer with all default settings which has worked perfectly for me in the past. Also the hotend has popped out the same spot every single time, second layer 3/4 the way through second layer. So I don't think the extruder is over feeding and the x axis is moving and the correct length and I did clean it out so there was nothing in the hot end. Also it is never jammed, if I heat it up and push pla through it, it extrudes freely. Any help is appreciated .