RSS Feeds show new Threads, not new Posts.
See these threads for methods to pull threads via javascript
:
[HowTo] Display your latest threads on an external page using an RSS2 feed
[HowTo] Display your latest threads on an external page using an XML feed
[HowTo] Display your latest threads on an external page using an RSS feed
To get new Posts, you will need to actually write some code to connect to the database on the other server. Usually, if the server is set up correctly, it will not allow an outside connection.