D.P.
I had that happen a couple of times. I think the milk somehow dried inside the opening to the duct and clogged it up. Standing in a hot shower with the water beads hitting the clogged duct, and then rubbing the spot with a hot wet washcloth did wonders. In fact, once after doing this I squeezed the duct with my fingers and saw the clog pop out like a tiny white cork - and then I shot milk 7 feet across the bathroom! Crazy - but oh, sweet relief!