URL Routing only works ...sometimes

I've been switching over to friendly URLs using the built in URL routing feature with ASP.NET 4.0
When I first sent them to the live server, they worked fine. Without changing any code in the subsequent days the all of a sudden it started generating 404 errors. I'm just wondering if there's something I'm overlooking, or if it's just something going on with my host / server.

public static void RegisterRoutes(RouteCollection routes) { routes.Add("TutorialType", new Route("tutorials/type/{type}", new PageRouteHandler("~/tutorials/type.aspx"))); routes.Add("TutorialCategory", new Route("tutorials/{typename}/{categoryname}", new PageRouteHandler("~/tutorials/category.aspx"))); } This probably doesn't matter, but the links are being generated like so...

<div class="designHea ...