Apple mobile devices not able to stream Youtube video

Dear All,

I am using Fortigate 110c, firmware version 5.2.7 GA build 718.

All of sudden, all Apple mobile devices e.g. iPhone and iPad can not stream any youtube video. Apple mobile devices can access/open the youtube apps and also can open the youtube homepage through device's browser but somehow for some reasons: not able to stream any video. It has no error message, it just keep spinning trying to reach the youtube server.

Oddly, laptop and all other Android mobile devices have NO problem when streaming youtube video.

Please share your ideas how and where do I start to rectify this issue. 

Thank you

Made