Your request needs to ask for a specific page or offset of the feed, and your server should handle this.
For the first feed, you'd ask for page 0. The server gives you items 1 - 30. At the app end, you keep track of the page you last requested. On the next request, you ask for page 1, and the server gives you items 31-60.
Alternatively, you can send the ID of the last item you have. So for the first request, you send nothing - the server gives you items 1-30. On the second request, you say "the last item in my current list is ID 14152" and the server gives you the next 30 items older than that.