Aluminum comes in all varieties. Some is very "sticky" while others machine nicely.
I have cut aluminum on my machine at 3" up to about 6" a minute .200 deep at a time. I run my router wide upen, and lighter cuts is better and easier on my machine(but I am impatient). I have had no trouble with the bit plugging up (1/4" carbide 2 flute endmill). I have been cutting 6061-T6 aluminum, which I beleive is known for it's good machining qualities. I have not used any lube.
What sort of machine do you have? If it's a wooden home made machine maybe a litle wd40 sprayed on the aluminum sheet before you begin? (since you can't flood the bit)
I know some aluminum is just too soft. You could try drilling a piece on a drill press. The soft stuff doesn't drill but just pushes the aluminum out the bottom and leaves it there, almost like you used a punch.
Eric