City destruction not taken into account in my attacks

[blafont]

Hello, in many of my recent attacks, I managed to destroy the ennemy's city but this was not taken into consideration by the game, ending up with one less star and sometimes even a defeat instead of a victory !
Is there any weird unknown unfulfilled condition (to me) that could explain this, or is it a bug ?

 

[oddin]

you didnt do it fast enough. There is a one min threshold to destroy either 50% of the base or the Town center. Failing to do either of these two, leads to one missing star,

 

[Snarlylemming]

They could also have the Forbidden City wonder which also has to be destroyed to achieve the town centre destruction star