Being a marshal does not give you ultimate power.. IMO it needs to be fixed. But lets not talk about that, lets rather talk about the subject here and I'll try to explain a bit.
Skirmishes: This is just a checkup. Do they join you in normal battles, but not in sieges?
Not joining in battle: I've heard that you have to have +5 in relationship (or higher) with the lords that follow you in order for them to help you in battle. This rule apply both in regular skirmishes and in sieges. You can get a lord with -20 in relationship to follow you, but he will not join you in combat.
Not following you: Several reasons. They could be distracted and start to follow a caravan or village farmers, or even bandits, you cant do anything with this. They could also decide not to follow you anymore. This happened several times when I was marshall and had a big campaign. And it even happened at the very start of the campaign, where all armies were fresh, no wounded or anything. And some of the lords who turned around had over +60 in relationship with me. I do not know why they turn back, but they do. Again, there is nothing to do about this except to turn around, hunt them down and order them once more to follow you.
All in all, you do not have full controll over your allied lords. This might be hard to fix too, it could be a mayor issue. And the developers might work on it and try to fix it for Warband (Warband will be singleplayer too, right...? I might be very wrong).