[M_D]

There are a lot of factors that can enter into the equation, but the method you are trying can and will work when done within certain parameters.

The first question, is what kind of aluminum are you working with, and what is the alloy? Cast aluminum is tougher than wrought or extruded, and some alloys are harder to work with than others.

The part needs to be free of oxide, which a good buffing should do. The parts need to be completely clean, or you may see uneven color. You need adequate agitation in the anodizing tank, the part needs to stay relatively cool without hot spots. Certain shapes and sizes of parts might require some consideration of the electrical contact point to ensure even current flow.

Without knowing more details, only guess can be made about your problem.