u/Alarming-Ad-1477

Apologies in advance for yet another rear derailleur question:( My new bike has had shifting problems since it left the shop. It’s only done about 200 miles, has always been skippy & jumpy in the middle of the cassette. I’ve tightened up the cable, which smoothed out the skipping, but now it won’t shift above the 6th cog. It’ll shift to the 9th with manual pressure, but I can’t physically tighten the cable any further. I’ve done a bit of bike maintenance in the distant past but this has stumped me. Please tell me what I’m missing here , I’d be very grateful for any help
Ps, it’s a 3 month wait to get the shop to look at it, otherwise I’d be straight back there.