The new posts should begin to fetch once you’re 600px from the bottom of the page. Your site is currently loading pretty slowly, which is why it’s taking so long to grab the new posts.
I ran your site through Pingdom, and it took 4 seconds for your server to download 25kb of data, which is very slow. Perhaps try contacting your host and ask them to update you to PHP 7. Then ask if there’s anything else they can see on the server side of things that would be slowing the site down.
A spinner would be a good idea while infinite scroll is loading new posts – I’ll get that added to GPP 1.7.
You could also choose to display the load more button for now – that would show people more posts exist.