Might want to also check the dies in the check maker. Depending on how one is making the checks it could make a huge difference. Is the check maker first punching out a disk and then forming it into a cup? Or, is it doing it all in one step? I used to work QA on terminals formed on high speed presses. The reason that the punch and form steps were performed in a certain order was mainly to eliminate material failures. Having not seen how commercial checks (or yours) are made I cannot give a comparison to the processes.
The alloy of the aluminum would also have some affects, as different alloys have different machining / forming characteristics.
Can you post some pics of the checks in various stage of making?