I've had sort of the same problem.. I'm a Marshall too and most of the Lords in the faction are even listed as Friends of mine (Apart from one of them who don't like me at all.. could be that I duelled him and won.. it was for a quest), and mostly some lords I have with me join the fight, but ever so often a Lord or two don't join.
BUT, you don't have to start the battle you know. When you "Enter" the city or castle you will see the text "Lord X joined battle on your side", so if no one joined just click leave and hold space (makes your party wait) for a while. Maybe the Lord was just about to chase after some other guys.
But mostly I get enough men with me in battle.. Just make absolutely sure they sort of like you and are not about to chase some other enemy.
EDIT: Also, sometimes, for no reason (especially when you just hang around for a long while, like when you construct a siegetower) the Lord will abandon his task to follow you. He might still hang around but his text will say "Waiting" instead of "Accompanying X". Make sure they are still on the task you gave them.