My bumper also sits lower after an OEM hitch install. No matter how many times I try to readjust, it doesn't sit any higher. The reason I believe is because the hitch cross bar and mounts replace the old bumper supports that hold the carrier bolts. It sits lower, so where the bumper rests on the new mounts, it's lower. I've read a couple other people on this forum with similar results, so I can assume at this point that it's normal.

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2000 Z3 2.3 Steel Gray
2004 X5 4.4i Jet Black