Fast Forward send me to beginning...

Hello everyone, I created a channel and I am on the testing phase. I uploaded some videos to my server Amazon S3, and they play fine BUT when I take a MKV and change it to MP4 with AVIDEMUX 2, it plays fine again, but when I try to fast forward it sends me to the beggining of the clip.. Any ideas?

They would be MUCH appreciated btw...

Thanks