Iphone 3GS home sharing working on some but not all video files loading .

I have successfully enabled home sharing on the mac and on my iPhone 3GS. If I start the iPod application I can browse the files on my Imac and I can play most of them.
Some of the video movies however (purchased ones) indicate loading and show two spinning wheels, but even after like 10 minutes nothing else happens and the screen remains black. I have access to the chapter list of the file, but selecting a different chapter makes no difference.
On my son´s iPod Touch 4G I can play all these shared video files from the Video App without any problem. The same home sharing account is used.
I am not sure whether the duration and thus file size might be a limitation my iPhone is unable to handle (maybe because of the wifi limitation 801.11g vs 801.11n), or whether there is anything else.
3GS iOS 4