Did you check your Sites.config and the default language for this website? There could be a difference between the language which is defined in your Sitecore languages folder and your configuration.
I had a similar problem with one of my projects where I changed the Sitecore.Context.Language to "nl" instead of "nl-NL". The glass mapper will return null, but Sitecore.Context.Database.GetItem will return an object in this case.