This maybe a datarate issue. It may stem form how you are utilizing the channels.
What Band are you streaming the video on?
You have to understand the IPTV support requirements- You may need to disable the low rates.
It's good that you enabled IGMP proxy- This will ensure that the wired infrastructure sends video traffic only to those APs that have subscribers. If you have the the correct band and Fixed Beacon Rates set correctly on the SSID profile. You might to enable the Dynamic Multicast Optimization (DMO) in the Virtual AP profile. I've seen this resolve alot of video multicast issue.
Virtual AP profile setting:
dynamic-mcast-optimization > enable
dynamic-mcast-optimization-threshold> 50