When is the last time you repotted them?
What type of soil/medium are you using?

African violets should be repotted regularly, which means once, or even twice a year for standards, more often for minis in small containers. Store bought potting mix is usually too dense - even if it says it's for African violets specifically. You'll probably want to add some perlite or vermiculite to make it 'fluffier' - the ratio you use depends a lot on your growing conditions (heat and humidity) and your watering regime. - If you find you water them inconsistently, maybe read up on oyama pots or wick watering and see if either would work for you (they require a lighter mix than 'manual' watering).

Having said that, the more inorganic matter you add to your potting medium, the more you will need to fertilize. Look into a balanced fertilizer that does not contain urea nitrogen.

Rachel's reflections is a great resource for African violet care information.

Good luck!

Karin

Here is a link that might be useful: Rachel's Reflections