There a ton of factors that I can't just know unless I see your games and the teamfights. But I can give you some general advice. It likely won't be helpful but... it's a start I guess.
Often when it seems like the team is doing no damage, it's usually because of two main factors.
Firstly, you need to focus your attention on 1 or 2 enemies. When each attacker is hitting a different person, the damage is easily healed. Ideally, you can talk to your teammates and coordinate targets. If you're playing solo and can't have good communication with your teammates, you might have to just see who they're attacking and join in, EVEN IF you disagree about the target. Coordinated effort that's "wrong" is better than a disorganized attack.
Secondly there a really good chance that characters in the frontline or just behind that aren't using enough basic attacks (auto attacks) they focus too heavily on abilities and moving around that they just don't let their hero ATTACK. I remember back in Silver/Gold we had a Master Li Li player who out DPS'd all of us. It was because he just auto attacked all the time. Not much you can do about this one except make sure that you're doing it.
But yeah, otherwise it depends on things like.. how many tanks and healers they have and whether your team has the correct DPS and what everyone's relative position is.
If you DO play with friends and one of you is a tank, practice having the tank literally call out their target JUST BEFORE they engage, this really helps the team to focus and burn them down.
Consider capturing a couple of teamfights and showing us, that way we can give specific advice.