In my experience, most Pokémon battles are lost because of one of these three things:

1. Your team, while made up of good Pokémon, only works as isolated powerhouses, not as a functioning unit.
2. Your team works so well with each other that your strategy was apparent, and someone exploited its weaknesses.
3. You just lost. Bulbasaur happens. Try again.

What if I told you I have a solution for problems one and two? I know you are curious. The answer to all your questions are just a click away. As long as all of your questions pertain to this specific subject.