There is a small chance that all trades of a villager get locked.
This is very unlikely, but happended to me today. I traded rotten flesh, gold, ender eyes, redstone, lapis, glowstone and XP bottles with a cleric and he didn't reset his trades afterwards.

Every single crossed out trade arrow in the GUI says: "Trade something else to unlock!", which is nonsense in this case, because you can't trade anything else.

I estimated, that the chance of this happening is 1 in 11000. Very unlikely, but still a bad game mechanic in my opinion.

I suggest adding a check to reset all trades in case you used up the last trade.