Rear derailleur needs a couple pedalstrokes to change gear 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.
Only 500 km from new?  Sounds like cable stretch where the inner cable increases in length from new as it beds in.

Did you use up your free service?  Most new bikes from shops come with a 2-8 week "tweakup" to fix minor things like this.  If you bought the bike off the web, then its up you to fix it.

Firstly check out your brake line - there should be some form of barrel adjuster, often at the deraileur end.  Try undoing it a turn to tighten the cable (ie lengthen the outer), and see if it makes a difference to shifting.



Sometimes they're on the downtube (this is more of a road bike thing)



You shouldn't need to adjust anything else on your deraileur, so no screwdrivers or hex drivers needed.

This is easiest if your bike is hanging up off the ground so you can rotate the transmission.  Don't put your bike upside down, things don't work exactly the same the other way up.

Also its probably time to lube your chain.