Whenever you post any query and expects some assistance from SCN, please ensure that you provide as much information as possible and avoid posting in one line and conclude that others would understand your issue.
Coming to your query, where is the issue? In Sale Order or Billing? Nevertheless, check in condition tab after clicking the Analysis button and maintain the missing combination in condition record.
Finally, before posting, please search the forum or Google it with the same text so that you will find this topic has been discussed many times. Take the inputs from there and try on your own. Still if you face any issue, update here.
G. Lakshmipathi