If they aren t you should replace it.
Mountain bike rear sprocket slipping.
I guess that the spring isn t hold the sprocket well.
The best is to bring the bike to shop to repair it.
The freewheel mechanism is contained inside the splined housing.
If not check cable housings chain and cogs for wear and see if the rear derailleur arm is bent.
Skipping can also be cause by a worn chain worn chainrings and worn sprockets step 2 remove the rear wheel.
To fix a skip in the rear derailleur shift your chain into the smallest ring on your rear cassette the hardest gear and the middle or larger ring on your front derailleur.
But if you want to repair it all alone first get your rear wheel off get the sprocket off and check the 3 teeth inside the sprocket they have to be perfect half circles.
Using a chain tool slip each of its ends through the links of your chain and check your chain against its wear indicators.
Look over the chain for excess wear or bent links.
If a link is bent either replace the link or the entire chain.
Disconnect the rear brake and undo the axle nuts step 3 remove the cassette.