Generally, we hear about this happening when people are sitting in a reclined position while pedaling, or pedaling extremely vigorously.
We suggest finding a seated position which allows your back to be upright at a 90 degree angle to your legs. If you're sitting very reclined, then there is a tendency for the machine to move because you are pushing forward with each stride. When your back is upright, then you will be pedaling with a downward force, instead of a forward force.
Also, instead of pedaling extremely hard on a low resistance setting, which can cause the Cubii to slip, try increasing the resistance level and then pedaling at a slower pace. This will prevent the Cubii from moving around while you pedal.
Additionally, you can also play around with the placement of your feet on the pedals.