I think you are right. The down shift lever doesn't even click. In my other bike when the FD refuses to shift, at least the lever clicks.
You should be able to easily check whether it's the shifter by applying extra tension to the exposed cable when trying to downshift. If it makes no difference, it's the shifter.
Do you mean, loosening the cable? I might bring it to my LBS. It's still under warranty. But it would be real silly if it turns out to be some simple fix....