Right now as far as I'm aware the client only lets you download replays of your last 20 games. I played 2 games 4 days ago that I'd like to get the replay file of but played a bunch of games since then so they're not in my match history. One was recorded on opgg but the other was not.
Another player in the game has only played 14 games so it's still in their history so the file still exists somewhere and is available for them to download from their match history. What is the technical obstacle that makes it so they can download the game but I can't?
This board is the best place to discuss the client with Rioters and your fellow players, although it won't be our main source for feedback resolution. Look to the other tools, like the Submit Feedback button in the client, for that. As always, please follow the Universal Rules and keep content focused on appropriate topics for this board.