Pre/post roll ads - MP4 is not playing

Hi,

We are using JW player in our site. Also we have integrated pre/post roll ads. The pre/post roll supports .FLV format only. When we use .MP4 format the player shows a black screen for some time and then plays the video. We don't know what is the exact issue as the player supports .MP4 format but the pre/post roll ads doesn't. Please help me out to fix this issue as this is an urgent one. Thanks in advance for your help.

Thanks,
Ram.