Message never makes it do device

[TWRaz]

I'm attempting to send a message to an Android device. This is the relevant content from the method I am calling

    GCM.key = "<my Google API key for server applications>"
    @project_number = "<the google project number for my app>"
    @reg_id = "<the registration Id for the android device>"

    def notify message
        options = {:collapse_key => "key2", :time_to_live => 3600, :delay_while_idle => true}
        data = { :key => @project_number, :key2 => ["Message Title", message ] }
        resp = GCM.send_notification([@reg_id], data, options)

The response I get from GCM.send_notification returns with a 200 status code and { success: 1 } in the body. But the message never makes it to the device. I'm clearly missing some piece. Can anybody shed some light?

edit: I have been monitoring the API logs on the Google Developer Dashboard, and see no activity.