Certified GKH Gardening Expert
It is either the fertilizer or the sunlight. If you use fertilizer, you may have given them too much nitrogen, which encourages leaves and discourages flowers. If you suspect this may be the case, add phosphorus. Here is more info on phosphorus: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm
A lack of sunlight can happen if a nearby plant has grown to shade the area. Check the area and see if the calla lilies are getting enough sun.