The picture below shows that the left side won't stick, and the right side no silk, this is caused by the unflat of the bed.
Here are some tips which will definitely help with the bed leveling.
1.Press on the button and click "Prepare", then select "Auto home", and the X Y Z axis will move to the original position.
2. When all axes returned to original, select turn off stepping Motor, the print head and the hot bed can be moved by hand.
3.Level the bed by twisting the bolt on each corner of the heating bed. try to make the gap between the nozzle and the platform is just as thick as a piece of A4 paper, which is about 0.1 mm.
3. Positively, this method can be applied to each type of machine.