I have resolved the issue thanks to David H. I had to define
#define _WIN32_IE 0x0900
I was under the impression that commctrl.h was defining 0x0500 by default (0x0300 is required for my functions), but it appears it wasn't.
If you do not define the _WIN32_IE macro in your project, it is automatically defined as 0x0500. - MSDN Source