@pgyurgyo
This game has been around for well over two years, so what happens is the following:
If you play in Diamond League, most players there already have full level 11+ decks.
If you play in Master League, most players there already have full level 13+ decks.
The game can't make opponents with lower-level decks appear magically. If they are not there (in those leagues), you will not be matched against them.
The game is designed to propel you to Master League through challenge matches (where you don't lose any medals), and through win streaks.
Bascially, what is happening is that you are playing too well for your unit levels. 😉
Your options are now either to: 1.) Spend huge amounts of money to level up your units 2.) to grind it out 3.) to quit. I recommend 2.)
When the season reset comes, you will be moved down in medals, and thus in leagues. DON'T go to the next higher league if you feel that your unit levels are too low to compete there. Instead, you can surrender games to intentionally lose medals, and thus stay in a lower league instead of advancing.
Hope that helps.