Home Garden Help Needed - Peppers

I have researched this problem and cannot find answers. I know there are a lot of gardeners in here, so if you can help please do so.

In our garden we have many peppers: sweet, bell, big bertha, marconi, jalapeno.

Generally, all peppers are doing fine. The plant growth is not a good as we would expect, but the plants look healthy and are producing fruit.

Except for the Marconis. They are stunted, wilting and dying. Some bore fruit but then never progressed. I have checked them day and night for insects: none. I don't think it is a soil problem either because the Marconis are not planted in one section. They are interspersed with other peppers. The other peppers are fine but a marconi between them is wilting. It's only the Marconis that are wilting. It's frustrating because the Marconis are my favorite.

I'm stumped. Any ideas?