Well, that would explain it...

Last week's muffins were yummy but flat, not really rising at all. I was perplexed but didn't put too much thought into it - until I was at Winco yesterday in the baking aisle when it suddenly struck me.

This:
is not baking powder.

Apparently I just reached into the cupboard and pulled out the first cylindrical container I found. No wonder one of my kitchen helpers said something about it being a funny can of baking powder. DUH! At least the mystery is solved. I will try to be more careful in the future :)