E-Sky Dauphin - a small technical intermezzo

Since some time this helicopter has no right yaw anymore. This means that I can't turn its nose to the right (tail to the left) anymore. I've posted this problem on a few forums about electric helicopters. Some nice heli enthusiasts gave me tips on where to look in order to find the problem. After close examination this is what I found :

The little copper wheel is attached to the motor. When it turns (motor on) it makes the large plastic wheel turn. There are actually 2 large wheels on top of each other. One to make the upper blades spin, the other for the lower blades.

This picture shows the black plastic wheel that drives the upper blades. The little copper wheel you see in this pictures drives actually the plastic one underneath the one you see.

The wheel for the upper blades looses contact at some point with the small wheel of the motor. At some other point it pushes against it.

This picture is seen from above.

I might be needing another plastic wheel or maybe a new inner shaft or probably both.

Some people suggested that one of the motors could be going bad.

Thank you for the suggestions.