Why did the fisherman stop trading?

When trading with the fisherman, first click the option to exchange raw fish for emeralds, then add the emeralds to the trading slot. Expected result: The fisherman gives me 6 cooked fish and consumes one emerald. Actual result: However, the fisherman's right-side trading slot shows nothing. But if I click the option to trade raw fish and emeralds for cooked fish again, although the trading slot does not change, the fisherman can trade cooked fish (or reopen the trading page and then put in the emeralds and raw fish).