1. The height of the idle nozzle is too low, turn it up. Please move the build  platform to the top of Z-axis, adjust the height of two nozzles according to the  gap between the print head and the platform, tighten the idle nozzle by wrench  and make it a little bit higher than the other one. 

2. The printing flow is too high, causing the actual layer thickness to be higher than  the original setting. In such case, after a while the object will touch the main  nozzle. Please turn down the printing flow.