Posting the XML would help. Context menu customizations will not appear in the <ribbon> group, but in a separate <contextMenus> group. Here is a sample of a couple of modified context menu groups:
<ribbon>
<!-- Ribbon mods go here -->
</ribbon>
<contextMenus>
<contextMenu idMso="ContextMenuChartArea">
<button idMso="ChartChangeType" visible="false" />
<button
id="CCCustom1"
insertBeforeMso="ChartSaveTemplates"
imageMso="ChartInsert"
label="Change Chart Type"
supertip="Change to a different type of chart."
showImage="true"
showLabel="true"
onAction="ChartChange"/>
</contextMenu>
<contextMenu idMso="ContextMenuChartAxis">
<button idMso="ChartChangeType" visible="false" />
<button
id="CCCustom2"
insertBeforeMso="ChartEditDataMenu"
imageMso="ChartInsert"
label="Change Chart Type"
supertip="Change to a different type of chart."
showImage="true"
showLabel="true"
onAction="ChartChange"/>
</contextMenu>
</contextMenus>