When shifting woes don't respond to normal tuneing, the first thing that I do is to check the derailleur hanger alignment. It's not uncommon for the hanger to get bent inward and that will definitely degrade shifting performance.
Shift your bike into a gear combination that makes the derailleur hanger point downward. They prop your bike vertically against something and examine it from the back. If the derailleur arm doesn't look like it's pointing straight downward, clamp a large adjustible wrench around the derailleur where it attaches it and gently bend the hanger on the frame it back into line.
If you try to bend the arm without the derailleur attached you risk ovalizing the bolt hole where the derailleur attaches. That's a very bad thing so don't let it happen.