Ko4evnik, the AI's behavior in some cases is a "mystery wrapped in an enigma." :) I've read about various instances of this kind of "stalling." I remember someone writing that they placed a pitiful group of peasants in front of an AI player's castle, and that coward didn't even dare to leave the castle! :)
Well, if an ally is being slow, so what? Who's in charge here? ;) As long as the enemies aren't being slow! :)
Right now, everything has continued to the point where I'm personally (!) manually opening mines for the AI!