Three of my best cities were producing wonders (United Nations, Rockn Roll, ...). Then I suddenly noticed that no production was displayed. When I looked on that 'city screen' (all cities) they all had "production 0" displayed. I checked them out and tried to make them produce the wonder again. It briefly appeared, then disappeared again. I checked if the wonder had already been produced by another civ., but no, no one. There was still good production going on, no anarchy etc. I just can't get them to continue to produce the wonders, but I can't see at all why not. Is this a bug or am I missing something!? Thanks