I have the feeling its one of the bug that happens after your claimant gets the throne. What happens is that your relation with everything that claimant holds is reduced back to 0, even your own fief. I have no idea how it happens but it does and so even if lords are close by they cannot join battle with you because they are technically 'neutral' and you need at least +1 relation with the faction in order for them to help you.
I don't know much about codes but you can just save a couple cilivians from bandits or help a lord in battle when he is losing, that'll get you back on the track.