villager discounts don't work

I was successfully saved 3 zombie villagers. All of them ended up getting jobs, and all gave me discounts on their products. This worked a couple of times, however, at some point they only accepted their normal price, even though the discount was still shown. An example of this was I needed to trade a villager (armorer) 8 coal for an emerald, but he showed a discounted price of 1 emerald. He would not accept the one emerald, even though the trade was shown. He only traded once I gave him 8 emeralds.