Can't figure out what I built wrong with the LEGO Sian gearbox

I've just finished building the Technic Sian, found a problem, and need some help. So, once every two gears, the engine stops moving, I've found out that that's when the top axle on the video is at the center, it is when it stops trasferring any movement to half the gearbox and the engine, I've checked the instructions thrice, visually compared gear setup with many on the internet and I cant seem to find where did I go wrong. Any help on where lies the issue would be super welcome! ...