I'm not accusing you of making it up, I'm only pointing out that it can only be one or the other.
The thing Is you cant play the game like you normally do and get them, you have to figure out which of your champions have a droppable chest and power farm in a game as well as protect your KDA.
The matchmaking in the game will adjust for skill level so the more you lose attempting this the easier the opponents you will encounter I think.
Some Champions are easier than other too. Bot lane in general is difficult because if your team mate is bad its a 1v2 lane, not to mention coordinated dives.
Also I think you posted elsewhere you mainly play aram. There are some Champions that I regularly get S with in that mode, but again, you need to Champions that have a droppable chest so random pick works against you, non-stop 5v5 also works against you since you might rack up a lot of deaths and CS is limited.
I only play aram for chest when I get lazy about it and I'm not capped at 5/5 chest available.