here is a tidbit taken from VirtualDub's site (a software you can use by the way to fix this problem ) on missing index information
"Missing or truncated index: This one is very common as it happens with any incomplete file. The problem here is that the offsets and sizes of the index can be recovered by scanning the whole file, which VirtualDub does when it detects a missing index, but the key frame flags that are required for seeking and decoding can't. VirtualDub recovers these by decoding the frames both in forward and reverse order and watching for artifacts; this drives some codecs nuts -- it's not exactly correct usage -- but usually it recovers the key frame information safely. If this recovery process is not done, then VirtualDub has to assume that only the first frame is a key frame, which makes seeking very painful -- potentially requiring every frame in the video stream to be decoded to retrieve the last frame."
This usually can happen if the encoder does not correctly include and index or does not put it in the file at all. So when a media Player tries and plays it it doesnt know where the keyframes are so it assumes there is only one keyframe and it is the first frame. The result will be that if you try to ffw it will send you back to the first frame of the video everytime no matter where you are in the timeline. If you use VirtualDub you can actually do a quick re-encode and it will fix the index and add keyframes.
hope it helps
