I found this answer on another website but I thought it might help you, the woman who asked the question was having a similar problem with her 4 year old plant hope her answer helps
Rose of Sharon only blooms on new growth, not old growth that was there from last year. If you prune them in the early spring before they start putting on leaves, they will grow and bloom in late summer on the new growth. Pruning a Rose of Sharon doesn't stop it from blooming, it encourages it, because it will grow and bloom on the new growth. Timing of the pruning is what counts. If the ROS had set buds already and then it was pruned, it won't bloom because the buds were cut off for that year.
That is why it is especially important to prune in very early spring. Don't worry about your bushes or tree-like ROS. Next spring just prune them back by 1/3 or less. Very early spring. No later than mid April. They will grow and set buds in May and June for their blooming again next summer which is probably towards the end of July sometime. Remember, don't prune off More than 1/3 of the plant. Don't fertilize the plant til July, if you must fertizile. You don't want to feed a stressed out plant, and by pruning it in early April, you are stressing it out.
It'll be fine for you, you'll see. They're very hardy.