I have Minecraft Pocket Edition on my iPad, and I have recently created a trading hall. I have multiple librarian villagers. When I hover my finger over the librarian villager and click "Trade" Nothing happens and the trade menu does not appear. If I click trade on a fletcher or farmer villager the trade menu appears. It seems that the librarian villager is the only one I can't open the trading menu with. In the screenshot I am clicking the "trade" button and the menu is not opening. This is also on a multiplayer Minecraft server.