I have a couple of Hibiscus plants that are struggling.

The are in separate areas of the garden and are grouped with other Hibiscus that are doing just fine. I've already lost one to similar symptoms.

There are no pests visible. The suffering plants have little new growth and are literally feet shorter than their companions planted at the same time. They have much fewer leaves and what leaves they have are tiny. If they manage to squeeze out a flower without the bud dropping off first, the flowers are smaller too, and are sometimes misshapen.

They get full sun and receive about 10 minutes of misting daily. I feed quarterly with Vigoro ornamental fertilizer.

Any thoughts? Your help is greatly appreciated.