I also have problems with code completion on MS Windows Vista 32-bit, using Mingw GCC 4.6.1 (tdm-gcc) with the 7th January 2012 C::B SVN nightly (from this thread), under various circumstances - mostly either with custom GCC features or with complex C++ template classes. Here's one test case (with custom GCC extensions) in which it won't work:
typedef struct __attribute__((packed)) _PSTRUCT
After typing the last dot after p, code completion box should open up, but it doesn't.
It looks like this particular case is a problem with parsing the GCC __attribute__ tag. Excluding it from the typedef makes the code completion work.
I also had an issue with code completion not popping up for certain complex (I think template-based) class declarations in C++, but sadly I can't reproduce them at the moment, I'll post a test case if and when I'm able to reproduce it again, it happened only once to me and I didn't bother too much cause I considered it to be minor enough.