Exchange 2013, a distribution group within a distribution group is not receiving emails.

Here's the explanation of what's going on:

"A" is an internal distribution group containing other internal contacts, and internal distribution groups "B" and "C". "B" is a working distribution group that contains all internal contacts. "C" contains all external contacts, who will successfully receive emails sent directly to "C". The problem is, when you send an email to "A", everyone in "B" receives the email, but no one in "C" does. "C" is set to receive emails from external sources and has correct SMTP settings as far as I can tell.

Is this problem related to the fact that "C" is full of external contacts? Is there a setting I'm missing somewhere that will allow "C" to receive emails sent to "A"?