Don't understand why (Total War: Attila)

Don't understand the mechanics of the game appeal ally to attack the city. Let me explain. In addition to the campaign goals I have my own personal goal(play as ants) to sklavyny and wends survived at least, and expanded as a maximum. For example, when sklavyny proschelkal at the beginning of the campaign your Karick from Kutikov, I later while they Horde wandered, gave them the ol. In gratitude, they colonized immediately adjacent Petrodava that I wanted to later colonize))) Well, not to take away the same))) sklavyny krasava, now 415 and they already Olbia, Petrodava, Hersonissos and even some capital in Asia Minor. But with the wends problem epic. 20 years have passed, and they are from Polotsk did not get out, never get out)) than When sitting with two shock armies with a total of 35 units. Three times I pointed out to attack one of the cities in Asia(the name I do not remember on top of Anakopia Tanais, but this city right from Anakopia), but they do not get out. Why? I even 7000 paid, so they went to war with Kartli and just captured this city, were fixed, and then I would help in the following captures. Moreover, I even the defense industry with slavename ripped, so they are the first wends captured the city, but those fucking sit. In the end, freaked out, ripped them, and the defense industry, and right-of-way :-) So what is mechanics? because sometimes it works and in the Events they write that ally calls. It's not random. Maybe because the Polotsk - edinstvinnyy their city and they fight the Huns and not come out until the war with the Huns anyway will not stop. Morett AI as a priority, sets the protection of the capital?