How do you get all four player to "fight"?

I play on android and i can only send one fighter per monster the rest are bricked off. so I am confused why it would be different for you.

Me too. This monster thing is a little wonky. I can't use the arrows to navigate between monster locations on that device. I see the bricks when i start a new battle, but it does not seem to matter, the game sends the first four to the same monster anyway. I'll try to pay more attention to exactly what i do on my next battle in 90 minutes.