"Insuffienct privilge" to add document to an ECO?

I've never encountered this before. We have recently created a new access program called "SC". Documents that have program access "SC" can only be discovered by some individuals. However, while testing this with a user with SC access, I noticed I can add normal documents to an ECO, but not "SC" documents. I have no idea what I'm overlooking--I enabled pretty much all privileges for this user in regards to this access restriction (Read, Modify, Checkout etc.) yet I keep getting "insufficient privilege" errors when I try to put the doc on an ECO. What am I overlooking?