A wide variety of things can be responsible for such phenomenon, most of them are not related to the gearbox. To find out what causes this, the nature and cicumstances of the skipping/slipping have to be identified exactly.
1. If this happens frequently and reproduceably in small gears and on inclines, it is very likely that the chain or belt is skipping. Check the correct tension of chain or belt and the wear of the complete secondary drivetrain. If a belt or chain tensioner is used, incorrect positioning and/or adjustment are possible reasons for skipping. On full suspension bikes incorrect adjustment of the rear suspension can cause noises or belt slipping.
If a belt tensioner is used, a snubber below the rear belt cog is mandatory. It can only work well if it is correctly positoned on the rear entry point of the belt on the sprocket with a distance of 1-2 mm.
When replacing a part of the secondary drive, all components should always be replaced (chain/belt AND sprocket front and rear) to prevent slippage due to uneven wear.
2. If, on the other hand, the slipping occurs irregularly and independently of the load after a shifting process or when starting pedalling after freewheeling, the freewheel mechanism of the rear hub is most likely responsible. It is a clear indication if the chain/belt continues to run when the slipping occurs.
3. It can happen sporadically that a shift pawl does not fully engage with the gear after a gear shift or when re-engaging after freewheeling. This is not noticeable at first, as this condition can be stable for a few pedal revolutions and only when the load changes slightly, the shift pawl will jump into the next toothing. This is usually associated with a short, clearly audible noise and with a free crank travel of approx. 15°.
This rare phenomenon cannot be ruled out 100% in pawl-based systems such as the Pinion gearbox. However, the gearbox is designed for the resulting loads and will not be damaged by these events.
If the cause is still unclear, use the following checklist as a troubleshooting aid.