Command F5 not toggling Voiceover!

All: I recently encountered a very odd experience on a clients iMac and am just wondering if anyone has encountered this before. Basically, once the user was logged in, I was not able to toggle Voiceover using Command F5. At the login screen or while logged in via Safe Mode, Command F5 worked as expected. However, once the user was logged in, Command F5 just caused the computer to beep. I contacted Apple Accessibility support and they say it is probably some application which is installed on the Mac which is preventing this keystroke from working. Obviously, it is not an Apple app. I'm just wondering if anyone else has encountered this before and if you have any idea what Mac app wold cause the Command F5 command not to work. Any suggestions truly appreciated. Thank you. Jim