Do you have a SAMPLE video on the player?
Can you send us a URL link to where you bought the player from or was it from an actual store?
What you could also do is download a whole lot of sample .avi video files from here:
http://mympxplayer.org/sample-rockchip-videos-with-different-screen-sizes-df1014.html and see if they play on your player.
If you can post more information about your player, some photos etc... that would be more helpful.