First of all, the obvious answer: Have you checked
Tools-->Options-->Text Editor-->C++
and made sure the "Statement completion" options are enabled?
Then, I am sorry to say that Intellisense is somewhat shaky in earlier versions of Visual Studio, they pretty much did a complete re-write in 2010 for this reason. This is particularly evident with C++, which is harder to parse than languages like, for example, C#.
Intellisense also breaks easily, forcing people to do all kinds of stuff to fix it (i.e. deleting corrupted NCB files etc.). There is a checklist available here for Visual Studio 2005 that is also still valid for 2008 which can help you fix things.