Help with winning more

Hey guys,

I've been working hard to improve my game and gotten a very good understanding of most things, but there's one that just doesn't make sense to me. Maybe someone could explain it to me.

I just can't figure out what it is that I do so much better with Sona than Nami that makes me have so much better of a win percentage with Sona. It seems to me that the early game is not much of a factor. But when it's about even going into late game with Sona, I almost always win. But with Nami, if it's about even, my team almost always seems to lose somehow. I have some ideas as to why:

1. My teams are just so dumb with Nami. They always just go into the stupidest situations and die almost instantly so bad that I can't even really help them much, and repeat so many times that my team loses. This was actually what I thought for quite a long time but now I've lost so many games that I'm having trouble believing my teams are just that dumb every time like that. Although this may still very well be the answer, because I'm always so carefully trying to think of what's the best thing I can do with Nami that I rarely give instructions to the rest of my team like I do with Sona. So it might just be because I never tell them to group up so they end up throwing all these.

2. There are better ways I could be playing Nami endgame. I know this is at least part of it, but a lot of things I feel like there wasn't anything I could do any my team just did something so stupid there was no way to save it. I know I could position better in several places but I figure this is something that will just take practice and not something I can just jump to being really good at. But a lot of the games there probably were ways I could have played in order to win.

Btw, a whole lot of the games I lost it seemed like I still had a very good kda ratio. In fact, my kda ratio with Nami is a .28 higher than that of any other champion I use. But when it comes down to some big team fight, I usually ult the entire enemy team and bubble at least 2 of them and use all the rest of my abilities on someone. But my team just seems to barely do any damage so much of the time and manages to lose the fight really badly anyways. I'm actually very good at landing bubbles, which is what a lot of guides seem to say is really important, and yet my team seems to have so much trouble doing anything with them, even with we're all standing right there.

Here's an example. Take my last game played: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1641183893/211539496

I felt I was very helpful to my team. I did 26.6k damage to champions, 19.1k damage healed, 40 wards placed, and 6 wards destroyed. I was 2/8/34 when my team had only 45 kills, meaning I was a part of 80% of them, which is a whole lot. And I had a **** ton of utility to help out my team so much. Aegis so they take less magic damage and gain HP5, morello to counter a lot of aatrox's lifesteal, (which he had a ton of) as well as some of Morgana and ez's spellvamp/lifesteal, rylai's so I slowed even more, and ardent censor so my teammates would attack faster. I've even found I don't need chalice on Nami if you get other MP5 items like the Ardent Censor and Morello so I forewent that in order to get more goodies and it still did not help enough for my team to win the game. Even this is a game I'd like to write off as I lost since Jinx and Yi were too aggressive and died too much, along with the fact that my team's pathetic excuse of an idea of tanks are a Pantheon and a Yi. But I still feel like there are things I can do better and I'd like to know how I can win games like this.