Why Undo and Redo button are disabled in my Pivot
 

Can any one please suggest me why I am not getting UNDO and REDO
button on my Pivot table.
I just want to add that I have put a small VBA code which dynamically
changes the formula for a calculated field in Worksheet_calculate Sub.
How ever I have noticed that if I remove the VBA code I get these
buttons back.
But I can not remove this code as this is giving me one important
calculated field.

Can any one suggest me how to get it back?