Problem with Apollo Gradient clunky and missed gear changes
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.