Need rear derailleur help...
 

I have a Giant Sedona ST - bought to supposedly free myself of the problems of cheap department store bikes. The problem is that I have FAR more problems now than I ever had with my cheap bikes! Mainly shifting. Currently, the rear derailleur will not shift into 7th gear, at all. I took it to the LBS where I purchased it, as I have several times over the year and a half that I've owned it, and their answer was, yet again, cabkle tension. They tightened the cables and sent me away. I tighten the cables myself, often. It *may* work for one or two actual gear changes, then it goes back to not shifting. Also, it seems that if the cable is really tight, that also keeps it from shifting into 7th. Is there anything else I can adjust?

It sounds like they've checked cable tension thoroughly; that's almost always the problem. If that isn't it, then your limit screws are set improperly, and don't let the DR move all the way over. The easiest way to check that is to remove the chain and cable, and move the DR by hand. The rollers should line up exactly with the inner and outer cogs at each end.

Or, you could have a bent hanger, or even a bent cage. Seems like your LBS should be catching this...

Hey, since the winter I've been having the same problem: not being able to shift into the smallest cog in the back. But on my bike, shifting up to the smaller cogs means releasing tension on the cable - at least I think it does. I also sometimes have problems going from 3 to 4 gear. I was going to let up on the tension a bit to see if that helps.

This strikes me as a good opportunity to start teaching yourself bicycle mechanics. A bicycle is a simple machine. If you have average or better mechanical ability, you can do this.

Go to parktool.com. It has on line instructions for doing most bicycle repairs and adjustments. Start at the very beginning and follow their instructions in sequence and to the letter. Don't succumb to the temptation to skip any instructions (that's what your bike mechanic is doing). My bet is that'll solve your problem.

Your shop must suck. Have you tried cleaning your drivetrains Clean chains, pullys and pivots can make a big difference.