My chain is slipping on my rear cogs when it up shifts and down shifts (usually the 2 and 3rd rear cogs). I been turning the twist tension knob at the top of the cable by the down tube(?) where it travels down and to the rear. This is a new bike with Ultegra components. It does not seem to be working (which is why I am here), do I need to focus on the limit screws on the rear derailure itself, or take it back to the shop. I'd really like to do it myself, does anyone have a good website or other advice?