Use plant growth regulator like NAA and potassium containing foliar fertilizer.

Mulberries are usually wind-pollinated, but poor air circulation/lack of pollen can reduce fruit setting. Some mulberry trees are male (only flowers, no fruit) or female (need pollen), while others are self-fertile. Please check if your tree is male, female, or self-fertile. Reduce nitrogen fertilizer, boost potassium & phosphorus. Maintain regular watering. If still poor, consider adding another mulberry tree (pollination support).