Make sure the file you are in is in C/C++ mode, Document > Select Mode... (in case if you are in an extension less file, or using an unusual file extension for C++).
What exact completion operation are you trying to do? Just hitting "." for auto-list-members, or hitting Atl+Dot to force list-members, or hitting Ctrl+Space to try to complete the symbol under the cursor, or something else?
Give me a complete example, exact keystrokes and settings (or default config is best), and I will find or explain the problem.