Derailleur hangers are designed take the impact of a fall, protecting the frame from being irreparably damaged. To do this, they're made from a softer alloy and often bend if the derailleur has taken an impact.
So, a bent derailleur hanger is often the source of skipping gears!
Derailleur hangers are unique to each brand... fortunately we always stock plenty of spares!
1. Remove the derailleur (usually done with 5mm hex key)
2. Remove the existing derailleur hanger with 5mm hex key and a chainring bolt tool.
3. Remove the old hanger and replace with the fresh one.
4. Refit the derailleur, re-tune and say hello to smooth gears once more!