How do I get all fields when merging excel to word doent?

I'm trying to mail merge from an excel file to a word doent. However, when I select the .xls from the 'select recipients' menu; I cannot see all the fields in the excel table (but in some older files I can...). Can anyone tell me what has changed between these two files and how I can get all of the fields in my .xls appear as selectable fields for the merge?