Lipo? Gel? or Stick Pack? Did you change out the clips to deans plugs?
How old is the Box?
This is what I've seen...1 motor was not working due to bad wire, so box was running off 1 motor instead of 2..
Also saw 1 motor fried, had to be replaced...Feel the motors after running if one is cold and one is hot, 1 motor good and 1 motor shot..No extra charge for the rhyming