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If the blooms have simply faded and are now dropping, the plant could be trying to go into dormancy. After blooming, calla lilies need a rest period. Give it time to go dormant and the plant should come back fine after this. Hold back on water do help it enter dormancy.

If the blooms were fine and then suddenly began dropping, this could be due to overwatering, which would also cause the leaf yellowing. Back off on the watering and allow the plant to dry out some before watering again.

If the blooms/buds are not opening fully, they likely need more water. Increase your watering. They should be watered at least daily and twice a day if the temps are over 80 F, especially if they're in containers. They do like sun but can tolerate partial shade. You can also add some phosphorus, such as bone meal, to the soil. For more growing info, this article will help: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm

It may not be getting enough light. They need lots of light. If you can get it more light, do so. You may also be giving it too much water. It needs to be kept moist but should not be in standing water. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm

There are many things that could be causing this. It could have too little water, too much water, pests, fungus or an environmental issue. I would recommend that you look at the standard care practices for calla lilies and make sure that you are doing them and, as a precaution, you treat the plant with a fungicide and a pesticide.

This article will help with calla lily care:
https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm

If they are yellowing after blooming, this is natural. They are trying to go dormant. This article will help:
https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/make-calla-lily-bloom.htm

If they have not bloomed, there are many reasons that this can happen. This article will help with pinpointing what might be the issue with yours:
https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm