Canon MF634 wont scan both sides

Greetings,

I bought a Canon ImageClass MF634cdw scanner/printer this year and it was working fine.  I use the Canon MF Scan Utility and it will work when scanning single sided.  However, if I click the box to scan both sides under Settings, then when I try to scan nothing happens.  It "thinks" for a moment then nada.  I've tried restarting and tried changing other settings to no avail.

Any tips?

Thanks.