The rear derailleur is the sex organ of your bike
Protect it, and treat it well, and it will work
Well for you all your days on earth
However
You may need to fine-tune your barrel adjusters
From time to time
Understand it
Know how it works by
Watching it as you turn the crank arm
While your bike is on its stand
Align it
Your chain should move smoothly
Around the cog and pulleys
As you pedal
Replace it
Make sure YOUR derailleur
Is compatible with
The shifters you’ve found for it