I'm certainly no expert but somebody told me this when I was in your position so I'll pass it on to you. Aluminum absolutely must be clean to get a proper weld. If it's oxidized it must be cleaned before welding. You can get the right kind of acid from your welding supplier. Make sure that the aluminum alloy your welding on is the same alloy that your welding with. Your supplier should be able to help you with that also. Use a stainless steel wire brush to clean your weld after each and every pass. Make sure it is a stainless steel brush and never use that brush on anything other than aluminum.

To address the problem of the wire burning back and balling up. I think either your wire speed is too slow or your heat is too high. If that's not the problem then make sure your tip is recessed at least a minimum of ⅛" inside your nozzle and when you pull the trigger make sure that you can hear the gas coming out just before you see the wire coming out. I hope something mentioned here will help. Good Luck!