Well, I tried besieging the fortress again with allies standing apparently closer, and then again for a couple of times - to no avail, unfortunately. Is there perhaps any other way to make the allies join me?
UPD: Reloading doesn't help either. Also, I'd like to note that there are several (5-6) allies nearby, and none of them join.
UPD2: Okay, this is unrelated to their proximity: they never join my battles at all. I think it has to do something with a buggy transition from being rebels. See http://forums.taleworlds.com/index.php/topic,158139.0.html for more details.