Finally captured this on video.   Sometimes feeding a sniffer a torch flower seed does not put it in love-making mode.   At first I thought I was going crazy, thinking perhaps I'd missed a hitbox or double-fed a sniffer, but finally observed this in action.   

In the attached video, when I'm on the stairs I feed the first sniffer a torch flower seed, it eats it, but nothing happens.   I feed the second sniffer and everything works correctly with it, but they don't breed because only one is in love mode.