FF or RW why doesn't it always work with some files

Fast Forward & Rewind....I'm sure this has been asked a hundred times but I can't seem to find a posting with the right answer.

I noticed that some files the FF or RW doesn't work. Why is that. I'm sure it's got something to do with the way the file was created, but is there a way around it.
So irritating when you want to pick up the second half of a film you had to stop. I'm forced to watch the whole thing through again.

I would sometimes look for another version of the same movie to download where the FF and RW feature works.

BTW.... this seems to happen more with MKV's although sometimes it occurs with AVI's as well.

[zip]

Can be multiple reasons:

- depends on your renderer and whether it supports it
- depends on if the file is transcoded and if the renderer supports time-based seek
- some renderers, even if they generally support seeking (byte based, on non-transcoded files), they refuse to seek in certain file types (or file created in a certain way) - this is difficult to diagnose, so the only way would be to reencode your file to a format you know is supported to seek in