I'm going to call this one a bug. And it's not a new one.
All of us have experienced it at one time or another, I think. It happens mostly with order delays, whether they are limited or historical. It can even happen in conscript mode. As far as I know, it happens on all size maps. It seems to happen more in PBEM games than solo play. I don't know if it happens in multiplayer, so if anyone has seen it there, let us know.
It can affect only one or two corps, or it can affect them all. Orders given during initial deployment always get carried out. Sometimes a corps will start moving several hours after the order is given, much longer than you'd expect from any reasonable order delays. Sometimes they sit there until the battle ends. I've seen them sit there for almost 7 hours once and the battle ended with them still sitting.
Occasionally you can get a corps moving by giving orders to one or more individual regiments.
It's bad enough when a corps doesn't obey orders, but sometimes it affects the CinC as well.
You can probably play well enough with visibility options set to Conditional, and I suggest Conditional Without Delay, but keep the orders set to immediate. Let us know if this helps.
Hook