I have the same problem. There is never any game modes going other than turf takover and vanquish. I want to play mixed mode because I get tired of playing 1 mode all the time. Either their matching server does not work, or no one ever plays the game. For $60 I am very disappointed by not being able to play mixed mode. The 1st PVZ still has full mixed mode games. I dont understand either.
Its slightly reassuring that i know that someone else has the same problem as i do. But we still need a solution to the problem.
Try updating the game.
