Microsoft Player Framework not playing local files

Microsoft Player Framework not playing local files

I am developing a windows 8 app in winjs and one of its features is to be
able to play local video files. So far I am using:
Windows.System.Launcher.launchFileAsync
Which works fine but obviously suspends the app and opens the video with
their default video program. I would rather keep the user in the app so I
tried to implement it using the Microsoft Player
Framework(http://playerframework.codeplex.com/). I know the framework is
referenced correctly etc because it can play video files both in the 'app
data' directory and online.
However when I try to play files within a folder that the user has picked
(with the folder picker) or the videos library that the app has declared
access to, the video fails to play. The app can however change the name,
create new files in the directory and delete files so I struggle to see a
permissions problem.
To get the path of the file I am using getFileAsync( and using the path
property and simply using that to set the src property of the MediaPlayer
control.
Do any windows 8 app gurus have any suggestions as to why it is not working?
Thanks in advance
p.s. let me know if any further info is required