As of late I'm having some trouble with my rear derailleur.
When I shift specific gears (most likely to happen to the middle gears), the rear derailleur feels like it doesn't have enough power and needs another shift to change gears (when it does it changes two gears at once).
And when it does change 2 gears I have to shift one gear down again for my desired gear.
I have tried cleaning the whole thing as best as I could. There was a bit of gunk in the derailleur, so I carefully removed it and also cleaned the cassette extensively.
As always I'm going to consult with LBS anyway but maybe this is an easy fix I'm not aware of, because I'm too scared to touch any screws/parts myself.
Parts in question:
REAR DERAILLEUR SHIMANO RD-M390, SGS
I can't figure out what cassette it is though, but it mentions 11-32T on the parts list so I think that's the cassette "size" and it's 8-speed.