Test it. Create a C project and try using auto-complete to see what choices it gives you. If it offers more, then you'll know that auto-complete is severly limited for stand-alone files. If it doesn't offer any more than what you were getting in the stand-alone file, then you know you'll need to start trouble shooting.
I say this, because it could be a variety of things. Your version of C::B, your OS etc. etc.