Originally Posted by
corrado33
Both of these are correct. Change your derailleur cable and housings. It only costs a few dollars and makes a world of difference if you're having shifting problems. Buy the "expensive" housing if you have a choice, it's only a tiny bit more expensive and does a very good job. (We have generic cable housings that aren't great, then we have Jagwire that's miles beyond the generic stuff. I'm sure there is better stuff as well.)
The cables are virtually brand-new. The entire bike is about 3 months old. I know it's possible there is some cable friction going on, but it's such a new bike, I'd be pretty surprised if it were the cables... I'll check anyway.
I think there's a misunderstanding about the "few turns" on the barrel adjuster. I have to turn it a few turns in order to get anything happening between the 16t and 19t cog. I do 1/4 turns at a time, so basically 3-5, 1/4 turns on the barrel and it will finally shift up into the 19t. However, it won't shift down now... I then start making 1/4 turn adjustments clockwise and it basically takes 3-5 of those in order to get it shifting down into the 16t cog now, but I'm right back to where I started.