El Capitan places the Courier Final Draft font in a location that Final Draft 8 -- an old program -- doesn't know about. This can result in formatting and pagination changes.
If you have installed FD8 on a 10.11 (El Capitan) computer or have upgraded a computer running an older version of OS X to this newer version, you will need to manually install the Courier Final Draft font. Once this font has been installed to a location FD8 can access, your scripts should again look like what they're supposed to look like.
If you have saved your script in another font and want to change it back to Courier Final Draft, follow the directions in this article:How do I change a whole script to another font?
If you still have issues with FD8, do a thorough uninstall and reinstall of the program. Directions are here: Uninstalling Final Draft 8 from a Macintosh computer.
The full Final Draft 8 installer is available HERE.
This font issue does not occur in Final Draft 10. Information on upgrading is HERE.