Gears shifter kinda working
Here are some things that can cause that:


The cable needs to be adjusted either at the derailleur or at an adjustment on the cable. This is the most common cause.
The chain, cable, and/or derailleur may need to be lubricated.
The chain my be worn out (stretched). You can check it with a special tool, at a bike shop, or by seeing how much you can pull off the large sprocket.
The cassette (rear sprocket) may be worn or have teeth missing.
There may be a problem with the cable routing, or maybe it's about to break. Make sure then ends of the cable guides are in their proper place.
As Argenti mentioned, the derailleur or derailleur hanger may be bent.