Hello Mr. Barlow,
This can be cause by different situation: by re-numbering your model you choose a different numbering method that previously, or you used the method with "drawing number" and you mode detail from one sheet to another and it can be other issue as well. For this one, I will suggest you to contact us at the technical support by phone. If you are not sure of how to reach us you can contact your sales representative.
Regards,