Probably nothing wrong with the derailleur or shifter. Simply shift the brifter into the small chainring setting (clicking the smaller button behind the lever several times, while pulling lightly on the cable with your hand if you have to), loosen the cable locknut, take up the slack, and re-engage the cable locknut. The main thing you want to remember is to leave a little more slack in the cable than your intuition tells you.
I made the mistake of making the cable too taut not once, but twice.
It may be that you had the brifter in the big chainring position when you engaged the derailleur's cable lockring, but it is more likely that the cable just needs a little more slack.
Good luck and let us know how it turns out!