Here is a page that shows why this can be different for different people.
Update Fields VBAGoing back to the article on the way Word thinks, each compartment is a container for different things. Fields could possibly go in many of the different compartments, depending on how (and where) you inserted the field. In the code SuperBoyAC first posted, you loop through all Word ranges, look in each range's Fields collection, and then cycle through those fields.
In the first reply of the webpage I just linked to, the user loops through each field in the Document's Fields collection... which is different than what the first code does.
It's all about object hierarchies, and I agree can be very very confusing. However, dont go bashing VB for this problem!
This is a "problem" with any object-oriented language/application. You can bash MS for making the Word Object Model so complex, but VB/VBA is not at fault here. If you try to script Word through any com enabled language you will have to use the same types of object references.
Without your document in front of me I dont know which container the fields you are trying to update are in. That's where the Object Model map can come in handy (I dont think the one I linked to above is the full map): it shows you the object hierarchies and where each piece can be located.
Hope that helps explain... I doubt it solves the problem.