I am pretty new to road biking and I have a total newb question. I have a CAAD9-5 with the compact Shimano 105 12-27 compact rear cassette. On my last ride, I noticed some trouble shifting into the lowest gear - the chain just does not seem to "settle" in and there appeared to be some appreciable lateral movement of the chain, so I just upshifted into 1-2. I don't know if this is related or not, but when I was trying to adjust it after my ride, I noticed that my shifter will not allow me to shift into the lowest (largest) cog. Is there a problem with my rear derailer? Aren't I supposed to be able to ride that cog??? Thanks all.