i'm having the same issue.
RibbonDropDownButton CanExecuteCommand at certain scenario won't triggered, while other type like togglebutton are still working fine.
i dunno what really caused this, the only thing i can observed is after some ICollectionView.refresh on other things/parts, and ZIndex manipulation, the RibbonDropDownButton CanExecuteCommand then died.
but if i click on the ribbon bar/tab, it fired again
I tried the CommandManager.InvalidRequerySuggested() still to no avail
still seeking resolution