Well cables don't last forever. There are a lot of factors involved in cable wear. If you've always had the problem with shifting, then it's probably not that the cables have suddenly died. But today I was out for a 50mi ride. Around mile 40, I had trouble shifting in either direction in the rear. It would take 2 or 3 tries each time. Being pretty sure that the cable was near toast, I quit shifting the rear until I reached the shop, which was conveniently located about 2 mi from the end of my ride. I asked them to replace the cable. When they pulled it out, the end inside the shifter was shredded and would have snapped soon.

More commonly, you will get some gunk (from your bottles or the road) near the cable stops under the BB or grunge and dirt down in the shifter. Too much friction means poor shifting, and it will seem worse under load.

It could also be that your shop didn't tune it well from the start, which might mean a combination of things like a too loose cable and a too tight lower limit screw.