Make sure the rods on the Y-axis are level.  I was having the same problem .  I loosened the nuts and applied gentle pressure, raising the ends of the rods to the top and tightening the nuts.  Now, I can get all 4 corners to level.  I put a bubble level across the rods and made sure the front and back of the rods were level to each other.

See: https://3dprinterwiki.info/wiki/wanhao-duplicator-i3/duplicator-i3-calibration/y-axis-rod-allignment/