I had to matches where the enemy team would have two or four reserve heroes and they wouldnt be used, whats going on?
Quid ego in Ludo Navigraio Didici.
Joey is correct, if the user closes the game or it is disconnected on their end then none of their backup heroes will come out. I got lucky with this as my very OP opponent got disconnected and I had him down to just his ET (out of his first 3) when killed all of my heroes. Unfortunately for him he couldn't take down my tower before time expired. I'm sure he figured he would lose on a disconnect though.