I bought my first road bike recently, and I've noticed I can't change into the largest cog/sprocket on the rear derailleur because when I pull the shifter the chain is pushed into the side of the largest sprocket and not over any of the teeth.
If I (gently) use my thumb to push the arm (chain guide?) the chain will mount the largest sprocket but as soon as I let go with my thumb it jumps down to the second largest sprocket.
I'm guessing this isn't normal and somehow it hasn't been fitted quite right? Is there anything I can do to fix this or should I just take it back to the shop?