Hi archanagwda -
not to worry, this just means that it takes excessive time to buffer the video - more than 15 secs or more than the whole clip length is, and to protect user experience, RAF cancels the play for that ad. This is a rare event in production apps and in investigation has always turned out to be either
a) bad CDN (i.e. slow hosting of mp4) or
b) no low-bandwidth creatives in the ad (in VAST, multiple versions of the video are provided, so that the player can choose quality based on the network conditions)
your specific case might be that you are trying to play US-only ads from abroad (common for off-shore developers) and US ads servers are not configured to server fast worldwide. You could however host locally VAST files and change media URLs to point to local web server, that way issue won't happen during dev/QA