Like in real life, the strongest team does not always win every game. There is an element of luck involved but also it may be that your opponent has perfectly countered your tactics. Take a look at the Team Training, some tactics work better against weaker opponents and some against stronger ones. Also you can use your analyst to find out how attacking or defensive your opponent usually plays and adjust your tactics to counter that. There is much more that goes into a game than just being the strongest team but all great teams rely on a little bit of luck as well.