Your speed is slow, very slow especially for the feed you are using. It is likely that chips simply jammed in the flutes so you were rubbing aluminum against aluminum.
How fast can you machine run? Ideally you should be doing several thousand rpm and be using coolant. If you cannot go faster than 1000 rpm reduce both you feed and depth of cut and make sure you have coolant or lubricant.
Part of the problem with ball mills is that they have to cut all the way round the ball but this means the cut is getting closer and closer to the centerline so the peripheral speed gets lower. Right at the center the tool is pushing the metal aside rather than cutting it. Coupled to this is almost non-existent chip clearance.
An open mind is a virtue...so long as all the common sense has not leaked out.