Spots on my Agave "Blue Flame..." a Little Help?

I several agaves, three of which are the gorgeous hybrid known as "blue flame."  Two of my three blue flames, though, have recently become plagued with a few little brown spots on their lower leaves. 

Any idea what's going on with this?  There's nothing on the back of the leaves--no bugs or anything.  And so far it's just the lower leaves.  It's also only my younger blue flames (maybe 8-10" across) that are affected.  My big one seems to be doing fine...

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