Good growers have very high rates of ventilation to keep the plants dry and healthy. Poor growers have too hot greenhouses with too small or non-existent fans. Look for bugs. Oh boy, do they like tender bedding plants. A tip here is to casually wander over and check out the green peppers.
Green peppers are aphid magnets.
If the peppers have aphids, you can be sure there are aphids elsewhere too. Watch the fuschia and verbena baskets or plants, whiteflies and aphids both love these plants. Unless you are an insect-lover, leave any plants with pests on the bench. This is the time of year when temporary garden centres spring up as fast as dandelions on the front lawn. This is a tender topic and I confess my bias.