iOS AVPlayer rate property not effecting video playback speed

I have an iOS app (running fine on iOS 6 and iOS 7).

The app plays .mp4 videos that it pulls from my web server.

The videos play fine.

But if I change the rate to try and slow down or speed up the video, nothing changes.
The video still plays back at normal speed.

I have tried these various ways -

[player setRate:0.50];
[player play];

player.rate = 0.50;
[player play];

[player setRate:2.0];
[player play];