If you click the 'Copy' button after selecting a draft or sent email and nothing happens, you'll need to edit the email name/subject line of the email you're trying to copy. There's a limit of 100 characters for the name of an email, and the system will not let you exceed that amount. Please shorten your email name and then make a copy after these changes are saved.
If shortening the subject line doesn't work, try clearing your browser's cache and cookies, log out and log back in, then try copying the email again. If you're still unsuccessful, please contact Support with the name of the email you're trying to copy, what Operating System you have (i.e. Windows 7, Mac OS X 10.9, etc.), and what version of browser you're using.