Is it shaded more than the others?
Does it show any nutrient defficiciency such as yellowing leaves with green veins(iron defficiency)?
Have you fed it? Could be it isn't getting enough phosphorus for it to bloom.
Have you checked it for aphids. Several people I know here in Boise have said they have never seen aphids on there Rose of Sharon until this year. They have been extreamly bad.
Let us know a little more info and hopefully we can figure it out.
Dan Cook
Master Gardener