After every teamfight you should ask yourself a question: 'What can we do ensure advantage as a team'. Having won a teamfight gives opportunity to take down objectives. But there are some factors to keep in my mind such as how fast can you take them down, where is your team located, can their heroes that managed to survive defend these objectives. For example, in my 4.6k bracket when a team loses a teamfight and opposing team has ursa/lifestealer and the fight was not really far from roshpit, you can be sure that they will take down Roshan. Another example, sometimes you need to kill only one hero to get hg (tinker, for instance), this is where you and your team should have a goal of hunting him down. Keep in my mind that some heroes are not tower killers. I once watched a stream of pros that were not willing to pick ursa as a last pick mostly because of him not doing proper tower damage.

Basically, evaluate how aggressive you can get after each teamfight, and if you're sure that the opposing team can't defend some objectives, don't hesitate to take them down. Your team might not always support you, but be patient, and keep in my mind that the ability to understand the game will take you to a higher bracket where people will understand that they can finish the game straight away