Disclaimer: My post is based on the assumption of a new rider, or one who is not very familiar with basic bike maintenance. Don't take offense if you found this pedantic.
Tuning the rear derailleur is not a 1-time thing. One should eventually learn how to adjust the rear derailleur, or at least, learn to use the barrel adjuster 'index' it correctly whenever required. Some scenarios where your rear derailleur is off:
- Your rear derailleur's cable is new (new bike or just a regularly (say yearly) replacement due to rust or scheduled overhaul)
To fix this: New cable will slacken during first or second ride as it tends to stretch more. You will need to tighten the barrel adjuster. If it skips, it means it's not indexed correctly.
Follow this video guide will help you to learn this basic maintenance technique:
- The rear derailleur got impacted such as getting a bump from another bike or you laid your bike on the drive side.
To fix this: Check that your hangar is not bent. It should be straight and not curved inwards or outwards. If your hangar is bent, you either have to replace it or go to a local bike shop and get it bent backwards (which may or may not work well).
- The shifter is faulty. (Since your bike is new, I really don't think this is the case.)
To fix this: Go to a local bike shop and get it repaired or replaced the shifter.
Other possible causes:
- Rear derailleur pivot points are stiff due to debris build-up over time. Clean the rear derailleur and lubricate the pivot points.
- The rear derailleur's spring is no good. It is responsible for accurate indexing. If the spring is old, it might not return to the expected original position. (unlikely, since your bike is new)
- Cable is not lubricated due to old housing (unlikely for your case)
Is cheap no good?
I've built several bikes for myself and friends. The cheapest component I've used it's Tourney (8-speed) and the rear derailleur though cheap, it shifts perfectly. The shifting effectiveness of the rear derailleur it's ultimately based on the accuracy of the tuning. After 2 years, the Tourney rear derailleur on my gf's bike is still running smooth.
My experience of rear derailleur tuning
After changing the rear derailleur cable, my tendency is to really overtune (tighten) the barrel adjuster as much as the tolerance allows. It will slacken over time. More often than not, it will take about 2-3 adjustments for the rear derailleur to be in a 'stable' state whenever I put on a new fresh cable. Some folks will overtighten it when putting on a fresh cable, then assess the accuracy later on to make the the necessary adjustments.