I had the same issue about a month ago, I ended up having a broken shifter, but my bike was about 3 years old, and had many miles on it, so I ended up doing the upgrade a season early. I was going to upgrade to Ultegra over next winter, but I ended up doing it over the past few weeks. Not saying thats what you have to do, I would follow what others have suggested, and since your bike is pretty new, you are in luck and can get a warranty replacement.