Crown of Thorns

Question

Leaves are turning yellow and falling off.

Answer 1

Leaves turn yellow and fall off when the soil of the plant is too wet. Move your Crown of Thorns Plant to a warm bright location with good air circulation. Allow the top half of the soil of your plant to dry out before you water again.
https://www.gardeningknowhow.com/houseplants/crown-of-thorns/crown-of-thorns-houseplant.htm