There is a good reason why some heavier bullets have a lower BC than some lighter bullets. The length of a bullet is governed by the twist rate of the barrel. There is a limit that cannot be passed. In order to keep the bullet length within the parameters of the twist rate of the barrel, the only way to make it heavier would be to make the nose or the boattail, or both, shorter.
Given that a large part of improving the BC of a bullet lies in optimizing the shape of the ogive, making the ogive shorter and the BC worse, overrides the BC advantage of increasing the weight. The same applies to the boattail, to a lesser extent. Increasing the weight, therefore, does not always increase the BC.