There was a bug in one version where there were some situations when the custom provider wasn't called. We rewrote much of the code recently in build 1417, to make it easier to override, not just the detection, but the complete handling of different devices and templates.

Are you using a newer version than 1417? Are you using Webforms or MVC?

Einar Olav at your company has already used it in a project.

Could it be that you have a cookie on your machine where you have selected the desktop version at one time?

If there is a cookie where it is specified that it should use the desktop/normal version, it will never run the detection code or redirect to mobile.

The easiest way to disable the cookie creation is to override the method GetRelativePathOfTemplateFile on the template content class. The code inside that method looks like this:

 if (WAFRuntime.Installation.EnableMobileSupport && FilepathMobile.Length > 0 && UseMobileTemplate(true)) {
                WAFContext.Request.IsMobile = true;
                return FilepathMobile;
            } else {
                return Filepath;
            }

The UseMobileTemplate method call have a useCookies parameter that you can change to false. 

Since Template is a built-in content class, you can't override the GetRelativePathOfTemplateFile directly, you have to create new content class that inherits from Template, and add the override there.

We'll add a simpler option to override the cookie handling later.