What kind of begonias do you have? Are they inside or outside? What are the temps like where they reside? Do you allow them to dry some between watering? Have you checked for rot near the base of the plants? Is the soil soggy or dried out?

A picture is worth a thousand words if you can post one. It helps determine what kind of plant, what the pot looks like compared to the plant, how the soil looks, how the wilt looks, etc.