Because of their leaves, most people don't consider a Crown of Thorns to be a succulent plant, but in reality it is. They have limited water storing abilities, however, and often have to rely on moisture it takes in through its leaves so I would try misting your plant daily. You might also want to increase the amount of water you give it each time you water it.
Crown of Thorns plants should be watered once a week when it is in a growth cycle, but only once a month while dormant. Whether dormant or actively growing, never water before the soil has completely dried.
Crown of Thorns plants should have some direct sun each day, even if it just an hour or two.
Fertilizer: monthly with 'half-strength' liquid house plant BLOOM fertilizer to get it blooming.