Word 2007 mail merge from Excel spreadsheet

I've got one user among many who is experiencing the following problem. I can't replicate the problem purposely on any other workstation, and can't figure out why it happens on the one workstation in question.

Creating a mail-merge to an Avery 5160 label sheet (all straightforward). The merge list is in an Excel 2007 spreadsheet. When going through the process, after inserting the "Address Block", you "update labels" to prepare the template sheet for the merged records coming in the final step. Normally, you get the screen that looks like the "SUCCESS" attachment. But on the failing machine, it looks like the "FAILURE" attachment.

I can; of course, simply copy and paste the missing field across the remainder of the screen template before finishing the merge process, which will work, but I'm absolutely mystified about what is causing this behavior to begin with.

Anyone have a thought???