The Mavs have not been themselves, in certain areas. They're creating great shots with their offense, but they're not making them. Dirk is shooting poorly. JJ Barea isn't punishing the Heat like he did the Lakers and Thunder. They're allowing the Heat to grab 15+ offensive rebounds per game.
If they can get it back, return to form — hit the open shots, stop giving up second chance points — this series should soon be over.
Look at what they've done while trailing most time, game after game. Imagine if they could actually jump out in front and maintain a lead.