Need Help using Simulink / Arduino to control digital motors

I am using an Arduino Mega R3 board to control 3 Hitec digital servos (HS-5645MG) using an analog input. The program is in Mathwork's Simulink (2012b).
-Arduino powered through USB
-Motors powered with 5V external power ~800 mA and common ground with Arduino
-Input analog pin 4 using 'Arduino Analog Input' block.
-Simulink blocks used to process input signal and determine motor angle based on thresholds.
-Output digital pin 10 using 'Arduino Standard Servo Write' block.

The above setup is working great using some old analog servos that I had laying around, but when I try to do the exact same thing with the digital motors - NOTHING happens at all.

-If I use Simulink's Arduino Servo Sweep example model, the digital motors will sweep back and forth 180 degrees, using same power supplies described above. I am truly baffled as to what the problem could be!

Any ideas out there?