It's hard to tell without more particulars. If you have inspected for pests such as mealybugs or scales and it looks clean, the next likely candidate is root rot. If it is in a container, only water after the top 1/2 in. of soil is dry, and don't let water stand in a saucer under the pot. If it is in the ground, watering will depend on the local weather, the type of soil you have, and the quality of the local water--it may be better to contact your local cooperative extension or municipal water users association to get good watering info. Good luck!