Make sure the face of the hub and the matching inside face of the drum are clean and bare metal. If there is any paint or rust/dirt/crap on there then the drum won't sit true. Also, if it's a later style drum with just one screw then wind some wheel nuts on tight and see if it spins, as the screw might be be pulling it out of line. If thats the case, then you'll probably find that when the wheel is on and tight it sorts itself out.
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