Agree. There may be a kink or problem with a housing that's causing the cable to drag. Make sure it moves freely. Here's Shimano's installation/adjustment instructions if the shifter has a trim position for the big ring:

http://techdocs.shimano.com/media/te...9830606806.PDF

or the following if your shifter has a trim position for the small ring:

http://techdocs.shimano.com/media/te...9830606832.PDF