Extras, trailers not displayed in original quality on IOS devices

Hello, sorry for my poor english

I use a Qnap PMS and it works fine when using the web. Extras and trailers can be displayed at original quality without any problem. However when i try to play them on Apple TV or Iphone i have to downgrade its quality up to 320p to watch them properly. What is the reason of that? The network and speed connection is the same in all cases.
Thanks!