What versions of DEVONthink and OS X are you using? Is the account a Gmail account? (There were failures with some AppleScripts running with Gmail accounts in 10.9.0.)
FWIW, the scripts you mention work as expected (import messages, import attachments) over here with DEVONthink Pro Office 2.7.2 running under OS X 10.9.1, with Gmail accounts and non-Gmail accounts.
Also, FWIW, sometimes simply rebooting everything clears these problems.