can no longer capture cities

Hi, I`m playing Brave New World and I`m half way through the Renaissance era. My problem is that after another nation took a city from me that I had captured and restored their sovereignty to the original owner, I have attacked the city again to take it back but it ain`t happening. I`ve reduced their defences to 0 using trebuchets and they have no other units whatsoever, but when I try to stroll in with either minutemen or pikemen, nothing happens. I tried waiting one more turn, to make sure my units weren`t simply out of turns, but the outcome is the same. What should I do? I`ve taken many cities before and didn`t have this problem, using the same units. This is a game-breaker if I can`t take any more cities. Should I download some kind of patch if this is a bug or should I start playing from the beginning?

Many thanks.

I've never heard of that happening. By any chance did you have a mod active?

Screenshot? Mod? Is your unit starting next to the city (ie within range/enough movement to capture the city).