Answer: Reduced blooming can be caused by too much shade, or by poor overall health. Both of these should bloom their best if planted in a full sun location (direct sun all day long or at a minimumn for six hours including the hour of noon.)
They do well in average soil, but you could use a top dressing of compost each spring along with a slow release or granular fertilizer such as 10-10-10 per the label directions. Using an organic mulch in a flat layer about three inches thick over the root area can also help feed the soil as it breaks down slowly over time.
The hardy hibiscus (rose of Sharon or Hibiscus syriacus) needs a well drained soil and is very drought tolerant, however during extreme dry spells you could provide an occasional deep watering. The cherry is also somewhat drought tolerant, but also benefits from deep watering during dry spells.
As far as pruning, the cherry would not need much if any pruning. The exception is to remove dead or damaged or diseased branches, or to remove suckers that grow from the ground or below the graft on the trunk. The rose of Sharon does not require regular pruning, however many gardeners will trim it back quite hard in late winter to early spring. This stimulates vigorous new growth and should increase blooming as a result. This shrub blooms on new growth of the season, so do not prune it during the summer as this would remove flowering wood.
I hope this helps you trouble shoot.
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