I built this project and found that I also had issues with the build. After several rip-aparts and tests, I discovered what I believe many are messing up on this build.
The early steps of building the gearbox are crucial to later success.
Once you get the gearbox built, but before you have connected the motors, operate everything manually to ensure that the purple clips and the orange shifter are aligned as shown in the diagram.
Also, be sure that each of the shift rings is connected properly to the orange shifter. You can test this by rotating the purple shift actuator axle above and ensuring that all 3 shift rings move accordingly.
Don't forget the extra red gears at the end of the gearbox assembly. If you have extra gears sitting in a bag at the end of bag 1, you probably did something wrong.
The shifting works as follows:
There is a Park Gear at full clockwise for the purple clip. It will be at the bottom. This is also where you can operate the dump function.
1st gear is 90 degrees counterclockwise with the purple clip away from you.
2nd gear is with the purple clip straight up.
3rd gear is with the purple clip flat and towards you.
Once you've fully assembled the model and connected the cab to the rear unit you can test to be sure that everything functions correctly.
Be sure and do a recalibration if you changed anything in the gearing.
I had forgotten that there is a 3rd differential hidden underneath the yellow 3x3 / under the dump function gearing in the center. If you don't connect the cab or alternately connect a geared load to the center shaft, nothing will operate and you'll wonder why.
Hopefully, these more detailed instructions will get you unstuck and you'll be operating your truck in no time.