[AccessD] Need code help again Please x posted

I have a sub form where I need to do some logic checking. sfrmSampleSize
 
User either selects all pieces (ChkAllPieces), a yes no field , txtPieces, a
numeric txt box, (txtPieces) or N/A, (chkNA).
If more then one field is changed from 0 or not selected I run the following
code.
 
Private Sub ChkAllPieces_AfterUpdate()
DoCmd.SetWarnings False
    
    If Me.txtPieces.Value <> 0 Or Me.chkNA.Value = True Then
        
                Dim lngRetval As Long
 
               lngRetval = MsgBox( _
              "You have selected conflicting instructions." & vbCrLf & "" &
vbCrLf & "Please select one only:" & vbCrLf & "" & vbCrLf & "All samples, A
number of samples or N/A" & vbCrLf & "" & vbCrLf & "", _
               vbOKOnly + vbExclamation + vbDefaultButton1, _
              "Conflicting Instructions")
 
              Select Case lngRetval
                         Case vbOK
              End Select
  
    End If
    
Me.txtPieces.Value = 0
Me.ChkAllPieces.Value = 0
Me.chkNA.Value = False
 
DoCmd.SetWarnings True
 
End Sub
 
Problem : I can no longer set value of chkAllPieces to true. It
automatically changes back to unchecked.
 
Can anyone tell me why?
 
Thanks
 
Joe