not too sure, but i think it only happens with one or two video codecs. it may be that these codecs do not use keyframes (or the particular video is only encoded with the one keyframe at the very beginning) . these keyframes are a full picture in the sequence of the video images, and later frames are worked out on the differences between frames so the video can be compressed. ie just store the bits of the image which have changed, instead of the full image for every frame. if the keyframes are too close, then the compression levels drop and filesizes become huge. too far apart and the fast forward becomes very coarse, skipping huge sections of video. the fast forward has to find a keyframe to start from as it can't guess what differences have occured between the first keyframe and the place you are trying to skip to, so if there is only one key at the beginning of the video, then it can only go there. the avipreview program has a speed adjuster which will let you play through the video at a higher speed than usual, but it stiil cannot skip to a forward frame in these videos, that may help.
www.avipreview.com 
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