Has the BizTalk Server 2004 Mapper tool ever prevented you from linking a field in a map? Have you ever spent two hours figuring out why?
Check to see if the field or record has a constant value. If so, that is your problem. The following is from the BizTalk Server 2004 documentation:
You cannot link to a Field or Record node in the destination schema that has a constant value associated with it. On the other hand, you can link to a required Field or Record node in the destination schema that has a default value associated with it. Note, however, that when you test the map, the default value will be used.
Have fun…