Customised Ribbon not Re-evaluating as excpected

This cusxtomised ribbon has been built to re-valuate itself on changes of workbook and worksheet. However, the expected revaluation - see IsBtnEnabled code - no longer operates as worksheet change events take place.

The XML Code is as follows

<customUI onLoad="RibbonOnLoad" xmlns="http://schemas.microsoft.com/office/2009/07/customui">
	<ribbon>
		<tabs>
			<tab id="ACBAMapping" label="ACBA-Mapping" insertAfterMso="TabView">
				<group id="MapBooks" label="Parent Books">
					<button id="CreateMapBookandControl" label="Create New" size="large" onAction="CreateMapBookandControl" imageMso="UpgradeWorkbook" screentip="Generates a new map book and control sheet" getEnabled="IsBtnEnabled"/>
					<button id="AmendMapBookandControl" label="Amend" size="large" onAction="AmendMapBookandControl" imageMso="FormControlEditBox" screentip="Amends parent control sheet for added worksheets" getEnabled="IsBtnEnabled"/>
					<toggleButton id="TGlButHideControl" label="Hide/ Unhide" size="large" onAction="HideMapControlSheet" imageMso="VisibilityVisible" getPressed="GetPressed" screentip="Hides or unhides the mapping control sheet" getEnabled="IsBtnEnabled"/>
				</group>
There are several groups
			</tab>
		</tabs>
	</ribbon>
</customUI>

There is a standard module called "RibbonX" which includes the following code.

Public gobjRibbon As IRibbonUI

Sub RibbonOnLoad(ribbon As IRibbonUI)
   Set gobjRibbon = ribbon
   MyHideEvent = True
End Sub

Sub IsBtnEnabled(control As IRibbonControl, ByRef enabled)

    Select Case control.Id
        Case "CreateMapBookandControl"
            enabled = CreateMapBookandControl_Enabled

        (There are several Case statements)
        Case Else
            enabled = True ' assume everything else with
            'getEnabled="IsBtnEnabled" is enabled by default

    End Select
Call TrapApplicationEvents
End Sub

Sub TrapApplicationEvents()
Dim Sh As Object
Set xlApplication = New MyEventsTrap

'Assign the Excel Application object to the xlApp property
Set xlApplication.xlApp = Application

End Sub

Sub ReSet_MyRibbon()
On Error GoTo Escape
    gobjRibbon.Invalidate
    Exit Sub
Escape:
    Err.Clear
'    MsgBox "Ribbon Error"
End Sub

A separate class module handles the Excel events "MyEventsTrap"

Option Explicit

Public WithEvents xlApp As Application

Private Sub xlApp_SheetActivate(ByVal Sh As Object)
Dim wb As Workbook
Set wb = ActiveWorkbook
If MyHideEvent = True Then
    Call Test_IsMapBook(wb)
    Call Test_IsControlSheet(Sh)
    Call Test_IsMappingIndex(Sh)
    Call Test_IsMapCommentary(Sh)
    Call ReSet_MyRibbon
End If
MyHideEvent = True
End Sub

Private Sub xlApp_WorkbookActivate(ByVal wb As Workbook)
Dim Sh As Object
Set Sh = ActiveSheet


Call Test_IsMapBook(wb)

Call Test_IsControlSheet(Sh)
Call Test_IsMappingIndex(Sh)
Call Test_IsMapCommentary(Sh)
If MyHideEvent = True Then
    Call ReSet_MyRibbon
End If
MyHideEvent = True
End Sub

The application events initiates the ReSet_MyRibbon call, but the review of the status of the ribbon buttons - IsButEnabled - is not activated when moving from one worksheet to the next within a workbook.

I'd grateful for advice or a solution.