I thought I would dig this up because of an interesting thread on the TW forums:

http://shoguntotalwar.yuku.com/topic/49227

This implies that the deployment zone is always wrong in the campaign battles. No matter wich direction you attack from (or where attacked from) you deply to the south of the battle map. Thus the reinforcements are potentially filtering in from the right directions, it is that you are simply not in the right place relative to them...

I am going to try this out on the campign map by setting up to attack from the south with reinforcements to the south of me to see if that works as expected...

On a further note, if you are besieging you appear to deploy closer to the city (in the distance) than the defending troops do when they sally.