I have encountered the same problem in both FMP Adv 10 and 13.
When I import an Excel file into an existing Table, the last column of my Excel file is not always available for import. I have a work around: add a "dummy" column as the last column and the problem disappear.
When the Excel file is imported the first time, FMP works correctly. However, if I later add some more columns into the Excel file (and add corresponding fields in the Table), the last column will always not showing up in the list of available fields. If I add a dummy column after the last column and try modifying the script steps again, the last column and the dummy column will shown up.
Any suggestion on how this happen and is there any better work around?