Have you checked that the adjustment ring and top burr have been installed correctly?
When reassembling the arrow on the grind adjustment ring needs to line up with the one on the back of the grinder body. Then the top burr is inserted into the ring making sure the 2 white marks line up. Turn the adjustment ring clockwise while lightly pressing on the top burr carrier until you hear the adjustment ring release button click.
If this doesn't work get in touch with Sunbeam and ask for some shims that fit under the bottom burr. Apparently the nut holding the bottom burr in place has a left hand thread meaning it needs to be turned clockwise to remove it. This is opposite to what you would normally do.