Problems with Source sdk

I want to make an skybox but when the skybox block selected and I press Ctrl+H the skybox block deselected and every thing except the skybox block removes and not getting back on undo

and another problem with Vertex:
when im using it on cylinder almost always one or two sides(up and down) of the cylinder become gray and i can’t change the texture

what is the problem with that?
bug in source sdk?

ctrl + h is now the “quickhide” shortcut. You shouldn’t hollow in the first place, press “u” to unhide the objects.

Make your skybox out of brushes to fit around your level.

The second problem sounds like you are creating an invalid brush with your vertex manipulation. You cannot have concave brushes.