Which flex cable are you referring to? There are 2. On the H33D 1.1 and H43D uses 2 flex cables. 1 big flex cable runs through out the gimbal to send gimbal motor positions, power and video data and 1 small one on the gopro mount plate just runs video and power to the gopro.
Look at the mounting plate and see if there are lights - if its on red, then there is power going through the gimbal but it may have problems initialising.
Use the Phantom Assist software and see under the firmware tab that the H3/H4-3D is detected. It should give you a firmware edition if its communicating.
If this all fails, look up on how to reset the gimbal on youtube. You just metal scissors to short out 2 positions on the gopro mounting plate.