Hi Syed,
after looking at the service literature this unit is controlled by a canbus system so whatever the platform does it goes through the canbus and tells the TCON what it wants and then the TCON instructs the components selected to operate. So there is a white with red stripe wire and a red with white stripe wire that are related to the 2 speed circuit and brake release circuit that go from the ground box through the harness going to the 2 speed/brake release valve mounted on the frame on one end of the unit. This is a small block with 2 solenoids on it. The block should be clearly marked as to which solenoid is the brake solenoid and which one is the 2 speed solenoid. Check the power supplied to the 2 speed valve when high speed is selected and drive is requested and brake is released. The 2 speed valve should be getting the signal to change the fluid mode to put the motors in high speed mode.
If your signal voltage is low or not present at the solenoid then you will have to back probe the circuit and find where it is loosing continuity if this is the case.
There may be other reasons but lets start here.
If you want the service literature you can find it on genie's website, it is free to the public no charge. Just pay close attention to the model number and serial number when selecting the one that goes with your machine.