Currently, we have an issue with calling the Focus() method on the RadComboBox and RadSpinEditor controls. The FocusCommand is calling the Focus() method and since this is not currently applicable for RadComboBox and RadSpinEditor, these controls as not focused.
You need to implement a custom command following the command pattern from this Knowledge Base article: Reusing the command pattern implemented in TPF. Please note that in order to focus the desired controls you need to call the Focus() method on their RadTextBoxItems. You can get access to the items by using the TypeSelector.
The whole approach is demonstrated in the attached sample project. I hope this helps. If you have additional questions, feel free to contact me.