I wouldn't assume just yet. I know you are in a zone way above me. But I thought one of my four was dead (my Plum Crazy). It was showing no sign of life and the others were growing. Well lo and behold it just started pushing out growth so it's alive, just very much slower than the others. I know we have had an unusually cool spring and these types of hibiscus really want heat to perform, so that may have been it. The other possibility is many of the plants surrounding it have grown quite tall and may be shading it more than the others. Suffice it to say, I'd wait a bit before assuming the worst. Give them some light fertilizer such as kelp, or Yum Yum mix, or worm castings and hold tight for a bit longer.