I was going to say that often people will pick a "support" that can also "carry," because they hate the traditional supports, and this often causes warding problems late game because they're trying to finish their build and wards get in the way. However, if you have Sona late game, I'm not sure why she's not warding. As a support you always want to keep one slot open for wards (preferebly two for vision wards as well). That extra bit of whatever for a future item build isn't doing as much as a stack of 5 wards will do.
Also, everyone (except perhaps the AD Carry, and possibly the AP Carry if everyone is taking care of warding properly) should be warding. It's 75g. The support can sometimes spend upwards of 1500g on wards, any other role will spend 375g except possibly the jungler. That's one kill, or 18ish cs, buy the damn ward.
Have someone (other than the support) be in charge of reminding people to ward, maybe he'll have a 2 minute timer that goes off and then he'll pop up chat and say "Hey guys, remember we gotta ward."
The snowballing effect is first caused by individual skill - you won your lane because you have good 1v1 (or 2v2) ability. However the teamfight is a completely different monster. Without a good initiate, focused fire, displacement, etc. then every teamfight comes down to luck rather than skill. I've lost teamfights where the KDR was something like 5/23, simply because our team had no cohesion.
Take this as an example:
Top Malphite, Jungle Amumu, Mid Gragas, ADC Ezreal, and Support Alistar. You have to know these characters strengths. This is a beefy as hell team, and most of your damage will be coming from Ezreal with a little burst from Gragas. Late game, though, Gragas is mostly used for his ult to peel the team.
Situation 1 - Alistar jumps into the fray, they start targeting him, he knocks up a champ and then knocks them back (say the AP Carry, who is Fizz) in an attempt to kill him. Fizz laughs and hops back on top of Alistar, and he goes down. Meanwhile Malphite ults into their team and hits three of them - the tank, the jungler and poor Fizz again. Their ADC was smart and grabbed LW, seeing this tanky-as-hell team, as one of their first items. Their team gets scared and starts moving away from each other, at this point Amumu Q's in and Ults, grabbing two of them. Fizz's cooldowns are done, so he simply hops over Amumu and Malphite, ults Ezreal and QW's him, then E's back to his team. Ezreal dies. Gragas attempts to peel, but now the team has no damage, and their ADC simply destroys the tanks.
Situation 2 - Amumu decides "Hey, my ult keeps them in place for a minute or two, I'm going to initiate." He Bandage tosses in and (since he decided to be the main tank he went straight resistences and health) tanks the team until they start hitting him, where he ults. Bam, they're all in a nice, neat, comfy position for...wait for it...DAT MALPHITE! BAM! All of them are flying in the air, so Gragas can easily peel two of them away for Ezreal to RAPE, while Alistar stays RIGHT NEXT to Ezreal so Fizz can't do ****. Fizz tries to dive Ezreal only to be Knocked up and knocked back away, Ezreal downs him in a matter of seconds. Ace, the team lost no one.
Can you see this happening in your mind's eye? You have to realize your role, and your role will ALWAYS be changing depending on your team composition. Alistar won't always be required to babysit the carry during a teamfight, and Amumu won't always be the first one initiating.
Taking your own team comp into consideration, I would honestly get rid of Renekton and go with Xin. Also, Nocturne brings little to the team late game, especially with a team where you have little CC. All your CC seems directed at ONE person, except Xin and Sona's ult. Consider switching Nocturne to a jungler with mass-CC like Amumu or Malphite. All of your champs have incredible 1v1 capability but without a team-disrupter, you'll go down like flies every time. You could also try Kennen top and Xin jungle, with Brand mid. This would actually work quite well as Kennen's ult would place them perfectly for both Xin to get in and peel them to your team, as well as Brand to unload hell on them.
Also, instead of blind pick, try draft-mode. It's more competetive and (in my opinion) a lot more fun. Plus it'll get you ready for ranked teams. Also, it gives you the opportunity to ban champions that seem to disrupt your team's flow.
EDIT: I just realized something else, you have no tank. You need one. I'd go with Alistar instead of Sona, maybe, on bottom, or even Malphite. Tweak the team composition so you have these things:
1. Lots of health. At least one straight up tank and another tanky champ.
2. Lots and lots and lots and lots and lots of CC. The more the merrier.
3. Burst damage. Xin fits this role well. This is for downing priority targets.
4. Sustained damage. This is your ADC. Sometimes your top or jungler will fit this as well.
5. MOAR CC!!
6. AoE damage. Brand fits this well and can wreck teams when they're bunched up together with his pillar + his ult.
There's probably more to include but these are the main properties. CC is so goddamn important though, as well as someone who can simply take hits all day long. You lack both of these, and is likely the reason you're losing. Wards are important, but it's likely the other team isn't warding as well as they should either, so you can't blame it on that.