As a bronze player who recently picked up Janna, I just have to say if you're a good Janna, it really doesn't matter how good your team is as long as they know what damage is and you know who is doing the damage. I build Janna pretty much the same every game Locket, Crucible, Talisman, Sightstone, boots, and normally a frozen heart unless they're an heavy AP team. Between her Howling Gale knock up, the knockback on her Monsoon, the slow on her Zephyr, her MS boost on her passive, the AoE MR on her Locket, the shield on her locket, and the heal/cleanse on her Crucible (not to mention I always go heal AND exhaust on my supports) you aren't getting to her team, you shield whoever is either getting focused of killing things, it doesn't matter if they know how to use that AD or not, you Q their team, ult when you need to reposition a teamfight, you slow the bruiser chasing your carry. I can save teammates who find themselves in a 1v5 as Janna with this build, I can peel myself out of a 1v5 as Janna with this build. Give me anyone who does a decent amount of damage and I can 2v5 as Janna. I have a 54% winrate with Janna atm I average about 1/4/18 I've had a few bad games but a lot of my losses are games I still manage to do very well in.
Janna is one of the most forgiving supports for her team if you know how to use her. Since she can easily get over 400 MS without boots, she can rush a lot of other supportive items her team needs. She does really well with a tanky build (I use my Leo/Braum runes and masteries on Janna) because she's more about peel than damage anyway, Janna punishes a lot of lower Elo players who think they can set up plays because engaging on her team often means getting CCd to death to turn it into an easy 4v5.
I feel like the real problem is learning how to play Janna. Compared to someone like Nami or Sona or old Soraka, Janna is a lot harder to pick up in lower elos. Janna's tornado is harder to land, knowing when to ult can be tricky, how to use your abilities offensively without making yourself vulnerable to the 14 seconds your Howling Gale is on CD. If you know how to play Janna though, she doesn't require a duo partner to do well. if the adcs are trading giving your ADC a shield, or if you see an ADC throwing a skillshot buffing it with your shield is often enough to win trades.
Ult/Q also only rely on teamplay when used offensively. If you're peeling for someone with your R/Q you don't need any follow up.
To be honest Soraka has been the only support I've had any trouble laning against as Janna. Soraka isn't up in your face, you don't need to peel her off of anyone, new Soraka is going to go for the same kind of passive lane bully phase you're going for, except she's going to out damage you and she's going to heal for more than you. I've never had problems against Morgana, Thresh, Leona, Blitzcrank, Nami, Lulu, Sona, etc as Janna.