I've got a project set up, and all files mapped out appropriately in the Project Properties->Mapping Tab. On the Properties Tab, under the Files section, I have included a directory of files for Code insight includes:. Back in the IDE, if I look at the Code Explorer, the functions from those included files are listed. But if I am actually in the coding window, and begin the function name from one of the included files, it does not appear in the Code Insight list. If I go ahead and type out the function name, and then do Ctrl+Shift+Space Bar, the IDE reports Failed to find "<function_name>" function.

Am I missing something obvious?

If you have "limit scope to includes" (tools->settings->codeinsight), you need to make sure that all the included statements are resolved properly. Check Includes tab in the project properties and make sure they are pointing to right files.
Anyway, I'd not recommend you to turn "limit scope to includes" on.

I have the same problem and reported it earlier. If you do not limit the scope to includes it works fine, but you get what is expected and that is every little stinkin' variable or function that is in your entire project. Usually, a good portion of those are uninteresting in the current context and only cause confusion. The "limit scope to includes" option is a very good feature...if it just would work right.

It appears as that this is fixed in build 5028. At least it now picks up the variable names from files that were called in through include and require, which it didn't do before.
Thanks!