I've got a DT lining up at right CB on this play: http://mfn8.myfootballnow.com/watch/371#76759

Also, my MLB lined up at left CB on this play: http://mfn8.myfootballnow.com/watch/371#76740

Perhaps I don't have enough CBs in my depth chart? (I have 4)

This happens a lot in preseason if you don't have enough depth and your starters are rested - the #1 and #2 CB will be benched at the time you have set in your misc gameplan options, which means you probably dropped to just having 2 CB's - who are probably not in shape enough to stay on the field. Remember, you can have 60 players active in the preseason.

I have a TODO item to have the AI consider the depth of each position so in your case you'd keep all 4 CB's active even after the starters have been pulled, but it's not yet implemented.

Yep that makes sense. Cool, thanks for the quick reply.