Why not? What happens? What are you choosing? Could you provide some screenshots to show what you are doing?
How to take a screenshot on your Mac
Capture, Save or Record Screenshots in Mac OS X - Help Desk Geek
OS X Screen Capture Shortcuts
(⌘⬆︎3) Command-Shift-3 Capture the screen to a file
(⌘⬆︎⌃3) Command-Shift-Control-3 Capture the screen to the Clipboard
(⌘⬆︎4) Command-Shift-4 Capture a selection to a file
(⌘⬆︎⌃4) Command-Shift-Control-4 Capture a selection to the Clipboard
(⌘⬆︎5) Command-Shift-5 Invoke new screen grabber
Post the screenshots in your reply using the Image Insertion tool: