Leaf curl can be a symptom of insufficient or inconsistent watering, possibly made worse by high temperatures. Try watering more deeply and on a schedule and see if that makes a difference. Root problems or a too-small pot (if it is in a pot) could also be a culprit.
Another possibility is insect damage. Psyllids and scale insects can cause leaf curling. In this case you would also see other signs of an infestations, such as small bumps or white or black patches developing on the leaves, or you would see the actual insects. Here is more information: