Hi, Thea, thank you for your comment.
Some vegetables and fruits need male and female plants; most berries do. Blueberries and hollies are some famous examples of plants that will need your planting at least two, and in the case of hollies, you will need specific male and female plants. Some vegetables, mostly those in the squash family, have female and male flowers in one plant, and depend on insects or humans to engender pollination.
I understand that the flowers of peppers, tomatoes, and even eggplants - all of them in the nightshade family - are supposed to be self-pollinating, that is, both the male and female organs are in one flower. However, like my eggplants, and perhaps your peppers, sometimes the pollen just does not get from one element to the other, hence the little nudge with a paintbrush or a cotton swab. With tomatoes and peppers, whose flowers are smaller and lighter than those of eggplants, one can just tap the flowers gently or cause some movement to the plant, to shake loose the pollen and thus effect the magic of fruiting the flower.
While peppers like to be warm and well-fed, sometimes over-fertilization may zap flowering and fruiting power in peppers, causing tremendous and profuse leaf growth but no flowers or fruit. Use a low nitrogen fertilizer, preferably organic. (Nitrogen promotes great leaf growth which in turn retards the plant's ability to flower, and thus fruit.) Soil and weather conditions matter, too. Keep soil evenly moist; do not let the soil dry out. Avoid overhead watering, as that may wash away pollen from the flowers.
And then there is the matter of cross-pollination, but we won't get into that now, unless you insist - and then you'll have to let me know.