Its hard to tell from the picture, is the gap on vertical left and right border filled with a different color border or is it just the low quality picture?
If its a different color border in that gap I'd think you'd just look through the border graphics and copy yours over it. The quest buttons need a vertical left and right border because the buttons expand vertically to fit the quest response text.
Other then that I'm stumped. I'm not sure where in game quest responce buttons show up in the uibuilder but you could look there to see
what graphics are being used. Maybe the sourcerect is off.