Buffer Videos

i have a strange problum with Jw player, if i upload a video with 360 quality the video is playable from first moment, and if i upload a video with 480 quality video will start buffer until complete video buffer and then it start play, what is the real problum? any help will rally apriciated
Demos
http://yamiraan.net/test-4-2-2 video will start buffer and after complete buffer then it will play

http://yamiraan.net/hazrat-yousuf-a-s-movie-in-urdu-part-1-2-2
video play start play from first moment

http://ww.wp.xz.cn/plugins/jw-player-plugin-for-wordpress/