G29 (auto-level) can only be run after a successful G28 (homing
of all of the axes).First thing to check is that the Axis is mechanically as
level as possible. The distance between the hotend nozzle and all four corners of the platform should be as close as possible for the same z-height.To do so, make sure that the gantry arm is as parallel as possible
with the platform. You can check this by bringing the hotend to the front of
the machine and bringing the nozzle down to the platform. Then, by turning the
y-axis motor by hand (the one mounted on the gantry), move the hotend back away
from the front of the machine. Check to make sure that the nozzle remains at
the same height away from the bed. If it moves further away or digs in then
your gantry is not level enough. Auto Levelling will only compensate for a 3mm
deviation.