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One test you can do is see if the menu item is active.

In mail, select some text, then go to the following menu item:
Mail->Services->OmniFocus:Send to Inbox

If that menu item is highlighted, then you should be able to clip the selected area.

If you can clip an item using the menu.
You should be able to clip using the shortcut you assigned.
If you can't clip using the shortcut, then try assigning a different shortcut.

What operating system are you using?