Can you share a copy of your Fla via Dropbox, Box, or Google Drive?
I don't see anything obviously wrong based on the screenshot, but it would be helpful to see your actual file.
It seems that straight text won't work as a mask. Not sure why it works in the live preview.
You have two easy work arounds:
Select the text field and do a Break Apart twice, which will make it be vectors.
Select the text field and do Convert to Symbol. As a movieclip it works fine.
Wow, that was easy! Thanks so much, couldn't figure it out