the ai already expanded on all the space it had.
Oh, ok then. It has no place to expand but still not building economy? Did it have a lot of unfinished buildings or shortage of some materials?
I can't say that for sure because i didn't set the game on "all explored". I previously played the same map and found the same behavior, but I just assumed it to be an isolated case at the time. But in this case, I can only see that the number of buildings of the AI reached a peak 6 hours ago, then it decreased; while land size kept growing up to 4 hours ago. My scouts can see several patches of unused terrain, and though the ai has many buildings (some 260) I know more could be made with that land. The previous time when I played the same map with all the map explored I could see clearly that the AI only used maybe one third of its terrain. That time it was playing barbarians, this time empire, so it's not strictly tribe-related.
Just send me savegame next time, it is hard to say from description only