It sounds like you have two separate issues. For the gear not grabbing you may need to increase the tension of the idler pushing against your gear. This is most likely done by tightening the screw(s) adjacent to your drive gear. Furthermore the teeth on the drive gear may be too worn and not sharp enough to grab onto filament. Buy a new drive gear or clean the gear of plastic build up.
The skipping is most likely insufficient current. Increase the voltage on the stepper driver. If problem is still present a new drive motor/extruder setup may be needed
Prusa i3 Rework - Ramps 1.4 - E3d Lite6 - Full Graphic LCD Controller