Problem with Apollo Gradient clunky and missed gear changes About four months ago I decided to get back into riding. I bought an Apollo Gradient mountain bike. I know this is quite a cheap/basic bike but didn't want to spend a lot if I didn't stick with it. I have stuck with it and will eventually upgrade to something better but can't afford to right now.

Now onto the problem, the gear changes have never been particularly smooth but over the last month they have got worse. The gear changes themselves, while clunky, aren't a particular issue. What is an issue is that it sometimes misses a gear especially 3rd and 6th gears (so when I change from 2nd it will go straight to 4th or from 5th straight to 7th). This is more of an issue as you will think you are in the right gear and then it will snap back into the correct gear later on. This is a particular issue when cycling up hill as it throws my feet off the pedals. I've got massive bruises on both knees from where they have hit the handlebars at these times. This happens when riding on relatively smooth roads so isn't even anything to do with the type of terrain that I am riding on.

I have taken the bike to a shop to be serviced and they seemed to think it was ok but it is still happening. So with that information and the fact that I can't afford to upgrade my bike straight away what can I do to try and solve this issue with gear changes being clunky and gears being missed (or being missed then snapping back into them as I am riding)?
The first thing to check is that the rear derailleur indexing adjustment is good. I.e. the derailleur cage is aligned properly with the individual sprockets in each gear. If cage is moving slightly too far at each shift it could push the chain two sprockets over instead of one. I would expect a repair shop to be able to adjust indexing properly, and bad indexing causes the chain to rattle on the sprockets - which you don't mention - so this may not be the issue.

The next thing to check for is a bent derailleur hanger resulting in the  derailleur cage not being parallel to the sprockets.

Another cause of shifting problems is the shift cable sticking in the housing due to corrosion or contamination, although this tends to cause 'delayed' upshifts (from larger to smaller sprockets) as the derailleur spring cannot pull the cable through the housing.