Use a test record to check channel balance. Check azimuth while you are at it. Make sure the cartridge is aligned properly although from what you say, it probably is. Most likely with a little fiddling you can get channels balanced, although it is also possible that the cartridge itself could have a problem. If you have access to a microscope take a look to make sure the stylus is perpendicular to the cantilever. If it is not pretty straight, which is not as rare as you'd think, that can cause the problem you mention. Usually, though, this can be adjusted for my slightly twisting the cartridge body in the head-shell. Frankly, when this happens it is a PITA to fix as in essence you are deliberately mis-aligning the cartridge. If the cartridge is high value and from a reputable manufacturer it might be possible to return it to them for evaluation and repair if you can't dial it in yourself.