Sometimes the buildplate is too far or too close to the nozzle and the leveling knobs are not able to set the right position.
In this case you can set the Z level by moving the little metal clamp on the left hand side of the buildplate.
There is two scre holding the metal clamp and it can be moved up and down.
If you rise the metal clamp the buildplate will be more far from the nozzle.
If you lower the metal calmp the nozzle will be closer to the buildplate.