The chain skipping between gearshifts is perhaps the most common issue that pops up, and can be stressful when trying to concentrate on your ride. This can happen by your chain slipping between gears when you pedal, phantom shifts, shifting twice instead of once, or refuse to shift when you need it to. Stretched cables usually cause a skipping chain. The cable tends to stretch with usage, and will cause skips if the tension on the cable isn’t sufficient. For crisp, efficient shifting - the tension of the cables has to be adjusted.
To fix a skip in the rear derailleur, shift your chain to the smallest ring on your rear cassette and the middle/larger ring on your front derailleur. Press your shifter once. If the chain doesn’t move up a gear, you need to add tension. Twist your right barrel adjuster, which is placed on the brake laterally opposite from your brake lever—the brake cable passes through it as it enters your shifter. Unscrew the adjuster by turning it away from you. When the first cog shifts correctly, continue to move through the gears in your cassette, fine tuning with the barrel adjuster as you go. The same technique works for the front derailleur, which also has a barrel adjuster. However, the front derailleur barrel adjuster tensions in the opposite direction. Screw the adjuster towards you to add tension.