The feeds are stored in the cache folder like:
/forums/cache/rss5.xml
where in this case rss5 is forum number five.
So either your folder file permissions are incorrect for the cache folder.
Or you overlooked the enable selection.
GOTO Control Panel » RSS Feeds
check if the feed shows it enabled.
Also check
Control Panel » Permission Checks
and see if you have a okay for the cache folder
you can also click on each item in the check and it will expand to check the files that are in it.
