The iDJ's touch-sensitive wheels work by sensing your body's electrical conductivity compared to its own electrical charges. If the computer it is connected to, and therefore the iDJ itself is not connected to a wall outlet, sometimes charges can build up that make the touch-sensing ability less effective.
The quickest fix for this issue is to simply plug your computer into a wall outlet. The best option is to use a 3-prong plug so the computer and device are connected to Earth ground, however, any power supply should help considerably.
Summary: Connect your computer and iDJ to ground with a power supply.