Castle/Heroe/Convoy log jam at castle entry?

[love2winalot]

Hiya. On each of the Maps i made, it is only a matter of time before the following happens. This is for the computer controlled players in Tote.

1. Castle upgrades all creatues, and buys them.
2. Convoys, "2-4" arrive at a full castle.
3. Heroe, "also full" arrives at Castle.

Heroe can not get in as convoys block the way. Nothing in castle can get for same reason. Nothing can take the, "base level" creatures out of the convoy, so everything just comes to a screeching halt, and that faction is just stuck there until someone comes and kills them all.

I can't be the only one this happens to? How do you get around this, or prevent it from happening? Thanks for any help.