The issue may be because the nozzle is too high from the printing sheet. Calibrate the printer again and set a proper Z-offset may solve the problem.
Please refer to the following video to calibrate the printer.
If it is hard to find the best Z-offset, you can adjust it in real-time during the printing.
When the printing starts, swipe the screen from the right border to the left to open the hidden menu and then tap on the Z-offset option at the bottom. You'll be able to change the nozzle height now.
More questions, please send an email to email@example.com