How hard do you have to pull? Might it be that in the other direction your DR or the cable binding in a housing ever so slightly that it doesn't provide enough tension to pull on the shifter reel?
If this were a year or so old I might insist you check in the shifter for a frayed cable. But since new............. well you decide. I don't know how easy that is to get to on those type. And you didn't futch something up installing it? Maybe used the wrong cable?