Unfortunately, this can have several causes. These are epiphytes, meaning they usually grow on other trees, as semi parasitic plants. Therefore, they will require soils, such as orchid mixes, or those specifically made for epiphytes. Sometimes the issue can be caused by over, or under-watering, lack of light, AND too much light, or cool temperatures. Unfortunately these can be somewhat finicky plants. Are there any other symptoms? If you can include pictures, then it would better help me to help you.
For now, I will include an article for their overall care: https://www.gardeningknowhow.com/ornamental/cacti-succulents/christmas-cactus/advice-for-christmas-cactus-care.htm