PowerPivot seems to store a cache of what columns are in the database from my experience. Whenever I added new columns (e.g., to a stored procedure or table) I had to go the the Design tab and click Table Properties. That will bring up how you pull the table. Just click OK or Save (whatever the "done" button is there; I'm working from memory) and let the table load again. New columns should show up now.
Sounds buggy to me. If I SELECT * FROM TABLE, I'd expect new columns to be added in automatically, but using the approach above was the only way I got it working.