Button not visible in Custom Group

I have created a custom group, and added an icon button for the user to click on. For some reason, but button is not visible.

<?xml version="1.0" encoding="utf-8"?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
    <CustomAction Id="Ribbon.Documents.CustomGroupInExsitingTab" 
                  RegistrationType="List" 
                  RegistrationId="101" 
                  Location="CommandUI.Ribbon" >
      <CommandUIExtension>
        <CommandUIDefinitions>
          <CommandUIDefinition Location="Ribbon.Documents.Scaling._children">
            <MaxSize Id="Ribbon.Documents.CustomGroup.MaxSize" 
                     GroupId="Ribbon.Documents.CustomGroup" 
                     Size="LargeLargev" 
                     Sequence="10" />
          </CommandUIDefinition>
          <CommandUIDefinition Location="Ribbon.Documents.Scaling._children">
            <Scale Id="Ribbon.Documents.CustomGroup.LargeSmall" 
                   Sequence="230" 
                   GroupId="Ribbon.Documents.CustomGroup" 
                   Size="LargeSmall" />
          </CommandUIDefinition>
          <CommandUIDefinition Location="Ribbon.Documents.Groups._children">
            <Group Id="Ribbon.Documents.CustomGroup"
                   Sequence="10"
                   Title="Custom Group"
                   Template="Ribbon.Templates.Flexible2"
                   Image32by32Popup="/_layouts/$Resources:core,Language;/images/formatmap32x32.png">
              <Controls Id="Ribbon.Documents.CustomGroup.Controls">
                <Button Id="Ribbon.Documents.CustomGroup.Notify"
                      Command="CustomGroup.Command.Notify"
                      Sequence="15"
             Image16by16="/_layouts/$Resources:core,Language;/images/formatmap16x16.png"
             Image32by32="/_layouts/$Resources:core,Language;/images/formatmap32x32.png"

                      Description="Notification hello message"
                      LabelText="Notify"
                      TemplateAlias="o1" />
              </Controls>
            </Group>
          </CommandUIDefinition>
        </CommandUIDefinitions>
      <CommandUIHandlers>
          <CommandUIHandler Command="CustomGroup.Command.Notify" 
                            CommandAction="javascript: SP.UI.Notify.addNotification('Hello from my custom group);  " />
      </CommandUIHandlers>
      </CommandUIExtension>
    </CustomAction>
</Elements>

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