Post some more details of your type of bike, shifters, derailleurs, and number of gears.

Most MTBs have some kind of an indexed shifter. If it shifts one way fine, but not the other way, try adjusting the adjuster where the cable comes out of the back of the derailleur, especially if the chain is chattering a bit.

Make sure the "stops" are adjusted so that the chain doesn't fall off the inside or outside of the cassette.

The upper jockey wheel usually has a bushing and a bit of side to side floating movement which helps with centering, but too much float can degrade shifting. And, of course, extra flex in the entire derailleur.

The B-Screw on the derailleur should also be adjusted so that the jockey wheels are as close to the cassette as possible (in the larger sprockets) without rubbing.