After clicking it, I get an error that states:"Please add more constraints to your search. Searches that return all or most of the database are a bad idea".This thing happened to me a few times already, so I thought it might be broken. Especially that after clicking it the Search button above it also is automatically clicked and I'm moved to the http://withthewill.net/search.php?do=process page.I'm on a Zenfone 2 with Android 5, and browsing with FireFox. That error might appear because no new posts were to show, but I think I could get a different error then. When I'm on "full site" (don't know how the theme is called) the New posts button in the submenu works normally and it shows all new posts including the ones from my last visit.
Edit: I just tried it on the theme you previously mentioned, and it is working on it. So looks like only the Mobile theme doesn't work.