Riding over bumps, crashing and having your bicycle bumped can cause your derailleur to become misaligned over time. A misaligned derailleur can cause your shifters to fail to shift properly. To ensure a smooth shift, you should periodically realign your shifters. This should be a part of your basic bicycle maintenance to keep your bicycle in good running order.
Lock your bike into your work stand. Set your bicycle upside down on the handlebars and seat if you do not have a bicycle work stand.
Loosen the cable stops on the derailleur using an Allen wrench. Turn these bolts in a counterclockwise direction, but do not completely remove them. Pedal your bicycle by hand to allow derailleur to guide the chain to the smallest gear settings.
Pedal the bicycle by hand as you stand behind your bicycle. Check the alignment of the rear derailleur over the smallest cog of your gears. Loosen the high-limit screw -- marked with an "H" -- on the derailleur if it is pushing the chain to the left of the smallest gear. Tighten the high-limit screw if the derailleur is pushing the chain to the right. Use a screwdriver to tighten or loosen the limit screws.
Look at the front derailleur. Tighten the low-limit screw -- marked with an "L" -- with a screwdriver if the derailleur is pushing the chain to the right of the cog. Loosen the low-limit screw if the derailleur is pushing the chain to the left of the cog.
Shift the front derailleur to the largest gear. Watch the alignment of the derailleur over the large cog. Tighten your high-limit screw on the front derailleur until the chain just comes into contact with the derailleur. Loosen the high-limit screw until the chain no longer rubs on the derailleur as it is being pedaled.
Tighten the cable stops on both derailleurs with an Allen wrench.