Unable to open trade window with villagers

Starting today, I have been unable to reliably trade with villagers. I have a large village where I have done tons of trading. Starting today, when I approach a villager with emeralds or an item it holds what it would like to trade, but pressing on the trade button yields no trade window.

Sporadically (rarely) I am able to trade with a villager or two, but I have yet to isolate the instances where this works.