I use QtCreator to open CMake project. Some directories apart from CMakeLists.txt contains only headers files *.h and for those directories QtCreator in the project tree vie
There is a closed bug report about this issue: CMake project shows no files.
In that particular case the issue was with the chosen generator, Ninja, which is not well supported by QtCreator.
Please change that to "CodeBlocks - Ninja". Creator needs the CodeBlocks extra generator.
You should see a warning about that when hovering the kit (and the kit should have a warning icon in front of its name).
Using CodeBlocks - Ninja solved it for me too.
Overall, it may help to try up a few generators...