Update an item action not showing all columns from infopath form library

I am attempting to update a column value in an InfoPath form document other than the current item that the workflow is running on. I can specify my where clause in the update item action but the column that I want to change is not in the drop down list. The only list items are Name and Title. Any idea why? Should I not be able to see all my columns? I see all available columns if I look at a sharepoint list but not an InfoPath doc library.

If its not possible to change a form library document this way, what other options are available within nintex? 

Thanks

JP