First things first, check the voltage and make sure you are getting the full voltage at the lights, so if 12v I would set to 12.1v.
Then you are correct in thinking ground, as you are using the ground to data and power, double the ground wires.
I cannot see in the video, but I assume you have only got a short cable, but if it were long, then you could be introducing some data corruption.
Ok hooray fixed the problem. Things to remember when playing this game. Dont double wire your data wire, meaning running two wires for data. Its ok to run 2 wires for ground and for power, but never for the data. Also remember when you have ground and data coming out from the controller and the lights are getting power from another power supply which is doing its injections, run a ground wire in between both of these power supplies ie on the negative. The positive from your 3 pin connector on the first string can be hooked up to the postive on the injection power supply then into the output on the controller.