Metallic Powder Question

Hi all,

I've just experienced an issue with a metallic powder that fluidizes for long periods of time (1 week+) without being used or topped off with new powder.  First off, there appears to be a lot of fines in this powder that exhaust slowly out of the hopper as it fluidizes.  Second, aluminum oxide builds up pretty heavily on the insides of the hopper walls.  This is a non-bonded metallic that has a heavy aluminum oxide content.  This bed of powder would no longer apply normally and the final cured product was way off as well.  We started over with a new box of powder (from the same batch) and the issue went away. 

My question is:  Has anyone else experienced anything similar to this?  Is it possible for a bed of powder to change if it fluidizes for a too long?

Thanks for the help!