How old is the wheel?

Spoke tension holds the rim in line. If you're starting with a perfectly straight rim, perfectly even spoke tension will hold it perfectly in line. If your rim has been warped sometime in the past, the spokes on one side of the warp will need significantly more tension than the spokes on the other side in order to hold the rim in line.

The art comes in knowing how much spoke tension disparity is tolerable. If you're talking about a LOT of difference, sometimes it's possible to loosen all of the spokes, physically bend the rim back into line, and retension and true the wheel. Another strategy is to assume you have a wheel that's on the verge of failure anyway and replace the rim with a new one while you're conveniently in your shop.