What causes gearing problems?

Probably just a stretched RD cable. How long since the last sevice? 🙄

You could also try lubricating the cable (inner) before spending any money, see if that helps.
My mate used to tip the bike upside down and tip white lightning into the end of the cable outer

I'm almost positive it's a tension problem. You can always adjust it yourself, or you can bring it to a shop (for instructions, www.sheldonbrown.com)

I agree with the advice to look at your cables..adjust, lubricate as needed.

I'd also like to offer that keeping things clean can have a huge bearing on performance.

chain cleaning is good, but I'm a big fan of putting on some old clothes, getting out the rags and old toothbrush, and going to town on the scrud that builds up on gears, derailleurs and chains.

I have obtained jaw-dropping improvements in smoothness and performance after doing serious cleaning like this. With the scrud removed, it takes less effort to KEEP your bike drivetrain this clean.

Basically, if you can hear your chain and gears while pedalling (and not while shifting) I feel that cleaning is indicated.

(by 'scrud' I mean that pasty, hard substance formed when bike lube mixes with road dirt, then builds up and turns to something like cement.)

geardad

I'd like to add that a good, serious cleaning also helps shifting and chain performance.

what I'm talking about here is putting on a pair of old pants and ratty t-shirt, getting out the rags, the old toothbrush and WD-40 and going to town on the scrud that builds up on cogs, deralleurs, chainrings and chains.

I personally have gotten jaw-dropping improvements in ride, and shifting smoothness by doing such a deep cleaning, plus, the bike is much easier to KEEP clean once this has been done.

with careful application of persistence and elbow grease, you can pretty much return your drivetrain to a nearly new condition, and you won't believe how well it performs afterward, even on a so-called cheaper bike.

*Note: your goal is not to liberate bearings and races of their grease..you want to remove grime from the exposed parts, then re-lube them.

one indicator I use to tell me whether I need to do some cleaning, and whether I have done a good job is how my chain and gears feel. If I don't hear or feel them during pedaling (not including shifting, of course) then they're properly cleaned and lubed. If I do, I know I have some more work to do.

but you also can't beat good old eyeballing to check drivetrain cleanliness, and should do so after every ride.

geardad