There are rolls made by the commander of a unit involved in combat. If they miss, then it goes up the chain of command with each commander having a chance to roll to positively affect the outcome. For a regular unit with normal chain of command, they have a corps commander, army commander, army group commander and then OKH commander, so 4 chances. With something directly attached, they get just the OKH roll.
If you started with the 42 campaign, I think several of the German HQ's have minor Axis ally units attached. You can move them back to their respective HQs and put in German units instead. Also, AGN typically has a lot of space, so it could be possible to shift some of the northern AGC units to fall under AGN and then some of the northern AGS units into AGC.
You will eat penalties with units from different commands in the same combat. Sometimes this matters and sometimes it doesn't, but it is a fairly large reduction.
Hopefully all this helps.