There are a number of services that I offer at TJ Cycles. One of the services available is All Derailleur Systems, Repair and Adjust. You can see below a list of possible faults in this area of the bike and the repair that will be carried out with this service.
Chain Rub Adjust Back and Front Gears
Chain Dropping off Front Chain Ring Re-Set Limits on all Gears
Not Changing Smoothly Index Gear Changers
Unable to use all Gears