Well, I emptied my horn and blew in it tonight. My cheeks looked like a chipmonk but no air escaped through the horn.
Chairslayer mentioned "If you fill the horn too full, and then stuff the plug in, you can actually compact the powder enough that it won't flow."
So, that may be the issue. Perhaps in my overzealousness, I over filled it.
I didn't fill it quite so full when I returned the powder to the horn, we'll see if it gets "plugged" again.
Thanks for the advice in any case it's all good to know!
Zan