Another thought too could be your environment. Are you setting the Z height with the bed and hot end at temp? I had done it cold, printed once or twice, and then the next time the gap between the hotend and bed was visable. Adjusted hot, and it seems better but I am still keeping an eye on it. I have the older 10", so I have to manually adjust the endstop, until I can print out the parts for a micro adjustable setup.