Why is it that?

Why is it that sometimes you can completely oppress your laner to be almost non-existent in the game, roam and help your team get huge, but constantly lose? I feel like I rarely have bad games on Jayce, but I have such a poor win rate with him (49~50%). I lose lane maybe 1 of 10 games, but lose 5/10? It doesn't make sense to me. I am Silver elo, does anybody have advice? I'm not whining, I am genuinely curious why I can't win games on certain champs consistently.
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