You have to remove those little plastic circles, as they are screwed to the wall. Then you can untie the knot that holds the string to that little plastic circle, and lengthen the string, then retie the knot. On some of ours we had to lengthen them over an inch each.
One other note, make sure you lenghten each side about the same. There is a mechanism inside the blind pull bars that must be kept centered.
It is a simple job, just takes a bit of time and patience. But it is well worth the effort. Your blinds will be a pleasure to use, instead of a pain in the butt!