Can't Move Robber
On iOS, sometimes the spaces where I can place the robber don't appear. Exiting to the main menu and continuing the game fixes the issue, but just wanted to draw it to your attention.
Could it be possible that when you could not move the robber, another player still had to discard resources because they had more than 7 cards? Because you have to wait until that is done before you can place the robber.