PST sometimes being charged

I have some customers that usually are PST exempt because they are purchasing for production machinery & equipment.  My bookkeeper set them up as PST exempt in Sage, but this is not always the case 100% of the time.  Now that I need to charge PST on an invoice, Sage won't do it.  Changing the tax codes to GP from G doesn't fix it, nor does changing the customer profile to not PST exempt.  How do I fix this?