I figured as much. Is it due to the fact that aluminum oxidizes so rapidly?
I should restate that - most industry body fillers don’t stick well if you use the standard prep work for steel panels. You have to seriously rough up the surface the filler is to be applied to to give it something to get purchase on, more so than steel. Then you have to clean the area to about surgical standards. There are some primers that can significantly help in this area, though.
Then you have the problem where the aluminum in some vehicles flexes more than steel so the filler just cracks off if you don’t have the right stuff.