Feels impossible to get a chest with Nami

It doesn't matter how well I play, I can't seem to get an S- as Nami. I can go 3/0/15 and have the best vision score in the game, it's still just an A+. What else can I do as a support to get a chest if not going for most assists, high kill participation and doing way better than anyone else with vision? I've been trying to get a chest with her ever since last week, and it gets me frustrated that I only seem to be able to get A+ as Nami. Any advice? {{sticker:sg-soraka}}
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