I admit that I'm still confused about the barrel adjusters for my rear derailleur.

Very little seems to happen when I turn the barrel adjuster (near the horns). At least, I can't notice a significant difference in shifting, until the adjuster hits the limit.

I had this bike 'set up' recently, about 6 weeks ago, including cable changes - it was shifting perfectly (triple) until a hammerfest day where I chased some roadies up some big climbs, including sprints, and gradually noticed that the shifting was off a bit. I played with the barrel adjusters some, but after a tiny bit of improvement, I can no longer tell an effect even if I wind it maximally in either direction.

My LBS is happy to re-adjust this for me, but I'd appreciate Any tips before lugging it back to them for this minor adjustment.