svijayrathinam
Have a look at Airon's great thread on video codecs. Maybe the answer is in there.
https://forum.cockos.com/showthread.php?t=82459
I have various friends using video on reaper on a mac and it's been fine so there will be one small thing wrong probably on your system that might be causing it.
Try support email like EvilDragon says too.
Hope you get it fixed!