Left switch on gang light pair not getting power
These new smart switches are active devices, and power themselves between supply (their black) and neutral (their white obviously).  Therefore they care about the difference between supply (their black) and switched/lamp power (their red).  Your old switches didn't care about that.  

You need the smart-switch black (supply) to go to the bundle of black wires.  The smart-switch red (switched) must go to the lamp/fan, which are the red and black that go into the same cable.  

Remember, do not dim a motor.  Motors respond very badly to triac style dimming intended for lights, they need a different type of thing called a fan speed control.