Dancers who often come off releve, but appear to be mostly aligned, may need to strengthen their ankles and legs. If releving more than a couple seconds really tires out your calf muscles quickly, you need to try some therapy band exercises to improve the strength in your ankles and calves.
A great exercise is the simple flex and point with a therapy band around your foot; an exercise that strengthens both your ankles and calves. Also, make sure your standing leg is getting to a fully stretched position once you releve. If your standing leg isn’t engaged and maintaining a strong, straight position, it will be hard to do any number of pirouettes. At the barre, try some fondu exercises that go into releve, really feeling that your leg is straightening fully with your foot getting to your highest demi-pointe.