Importing supplier bills

I'm using SaasAnt to import construction industry scheme (CIS) bills our own system generates.  The product/service is set to CIS Labour standard (20%) and all the other fields are correctly mapped.  The import works fine and the bill is created but the "Less CIS" field does not update.   This updates automatically when I create the exact same bill manually.  It would be useful to understand why this field does not update for imports.  It's frustrating as I want to import hundreds a week.   Is there a workaround?