Apply a primarily nitrogen soluble fertilizer twice a month during the growing season. In spring, a foliar application of a fertilizer with chelated iron, zinc and manganese will encourage a dark green, healthy plant. I have many African violet plants. When I purchased them, they were blooming profusely and had small leaves. After about a year, they now have large leaves and only a few small blooms.
African violets can grow well but flower poorly unless light is sufficient.
Plants with long stems that tend to reach up are not receiving enough light. You will encourage more blooms by using a fertilizer that has a higher proportion of phosphorus. Your plants could be getting an abundance of nitrogen that promotes lush foliage at the expense of blooms. You will find your plants produce larger flowers if somewhat pot-bound. I brought back seed pods from a jacaranda tree I saw in California.