if this is on a Windows machine then it may be because Wowza does not have permissions to that shared drive. Wowza runs as a service normally, and doesn't have the same interactive access that you have when logged in.
A quick way to check is to run Wowza in standalone mode. Stop WowzaStreamingEngine in Services, and open a windows console and type this
This runs Wowza with the same access permissions as the person logged in. If this is satisfactory, then there is a way to permanently run Wowza in this mode. It's usually recommended to Windows users wishing to use hardware Transcoding, but follows the same principles here. It involves creating a Windows task using their task scheduler and is explained in this article.
Another option is to use MediaCache in league with a web server as described here. It is a bit more involved, but makes it very easy to access remote as well as local content without having to run Wowza as a standalone process. You just point your web root to where your content is and use the prefix (http is the example in the article) in front of the content.