Get the Park Blue Book.
Could be: r shifter, r cables/housing, r derailleur, chain, cassette, frame.
Derailleur, chain, and/or cassette could be worn, cables not sliding smoothly through housing, shifter issues. Check derailleur hanger alignment, frame alignment. If you're only getting it in the largest or smallest cog, could be the der stops need to be reset.
But yeah, ask the shop what's up.