Did you rename the map at any point? (This causes exactly what your screenshots show.) For building cubemaps, ensure your map is saved in dlc2 (prevents a Portal 2 crash bug) - check the last tab in hammer options. Then you can run "mat_specular 0" in the console to turn off reflectivity, "buildcubemaps" to actually generate them, then "mat_specular 1" to reenable reflections if the map reload didn't reset that for you.
If you rename your map after building cubemaps, strange things happen. So just remember to recompile every time you rename it.
This constantly happens to me as well. I build cubemaps and it looks great, and then I upload it and everyone complains about missing cubemaps.