By default, when you install a new font, only the current session is notified of the change. So if you're logging in to the server in a terminal services session (which seems likely) then the ASP.NET application (which will be running in a different session) will not see the change. When you reboot, the system automatically scans the font directory and "registers" all of the fonts in there into the current session. To "manually" register a new font, you need to call AddFontResource and pass in the path to the font. To make it slightly easier, you could make it so that your app scans the Fonts folder and calls AddFontResource on each file it finds there in it's Application_Start event. That way, when you install a new font, you can just recycle the site (e.g. edit the web.config file) and it'll re-scan all of the files.
the Telerik team