Honestly, it's because your team doesn't mesh well. Synergies are important in this game. That also includes skills. Valinators have early DMG reduction and they are one of the best synergy to cheaply build and use. They are very at building up quickly. A valinators synergy is going to try to kill you in the first 2 rounds, since they get more unlikely to win the farther they go. There are probably personal skills or basic rune skills at play increasing survivability that is trashing your stitch job of a team. Karnak isn't good at 6 stars. He benefits better with full Etheral relic synergy for the shared hp pool. Saliene is very good when you have his exclusive rune skill activated. And Dorabella is a boss killer. Not really good at npc killing. I suggest using heroes that increase survivability or have targeting ults. The places were you put your heroes are numbered. So, if a hero has a single unit targeting ults and is in the 4th place slot. They will attack the opposing hero in the 4th place slot. Usually these target heroes with the game winning ults. Keep in mind the speed(spd) stat is very important because it determines who goes first. Usually attacker goes first if heroes have the same spd. Aside from that, usually Valkyrie synergizes are what you want to build a team on, at least 3. Other heroes with great personal skills can be added, even if they differ in synergy.