You'll never get it to steer completely neutrally through the full rev range. Best you'll get is it pulling one way at low revs and the other at WOT. Then fine tune the trim tab to get it neutral at your normal cruising speed. If you're not getting near this the engine might not be properly centred on the transom or may not be trimmed properly but unless there is something grossly wrong with the prop I wouldn't expect that to be your problem.
Are you turning the trim tab in the right direction. Experiment with it at both extremes just to satisfy yourself.