Finding an opponent of similar power is a tough task, and there are several criteria of search. The game tries to match you with an opponent of roughly the same power, but if it doesn't find one in a certain time period, the criteria boundaries expand a bit, and the search starts again. This works the same for Clan Wars, except it's the summarized team power that's used to find opponents.
Here's the list of criteria of matchmaking search:
  • Power level of your army. The more upgrades your troops have (this includes stats, special abilities and skill tree), the more powerful opponents you'll face.
  • Your winrate (percentage of your wins and losses) on current power level. The more wins you score, the more powerful your enemies will become! The opposite works too: if you lose a battle or two, then the matchmaking will find a slightly weaker opponent for you.
Don't forget to upgrade your troops and tech when you have the option to do so, and you'll become stronger with each day!