This may happen if you have opened your template in a program other than Adobe Reader. My files will only work in Adobe Reader so please check this first!
Mac users can see here for specific help
Windows 8 users can see here for specific help
This may also happen if the text field you are trying to edit is an “Auto-Fit Field” where any text you enter will automatically resize to fit the space. You cannot manually change the text size in these fields.