WiFi speeds impact the download times of videos. Please report to your support agent what are the up/down speeds in the rooms that are buffering video files. (Use www.speedtest.net to measure)
Larger video files will take 5-15 minutes to download over the speed at 30-50 MBPS
Also, check to see if the green light appears on your menu screen (see picture below). The green light means the videos are still downloading. If the green light does not appear and the videos are still not loading. Unplug the AppleTV from power and then plug it back in.