Our team has been able to stack 10 cubes with a 1:7 ratio and 200 rpm motor cartridge. Maybe there is some friction in your system. We had a problem with the gears slipping but we resolved that by doubling up the gears. Maybe you could also try optimizing the physics of your tilter based on the thread linked above. Best of luck resolving this!
Do you think you could post a picture of your tilter geometry? 200 rpm 1:7 is really impressive
I have 1:11.6 100 rpm. still very fast but really torquey too.
you could swap to a 200rpm cartridge or a 600 rpm cartridge instead of changing the ratio. 1:21 200rpm works well though.
Adding another 1:7 was very unnecessary. With your existing setup (1:7, 100RPM cartridge), all you needed to get from 40Nm top torque down to 10-ish was a slight change to the arm and pusher lengths: arm from 13 to 8 holes, pusher beam from 14 to 20-21. Simple fix w/o changing or relocating the gearbox…
The best way to improve your tilter is to make the drive arm shorter and that will reduce the force on your motor and likely allow you to lift more cubes. You may have to increase the range that your arm can spin though.