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.
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.