The two most common causes of the symptoms you describe are (1) over-watering and (2) lack of nutrients.

Which of the two is the most likely? Posting a photo of the plant would help. Otherwise we need more details - containers or in ground, size of the container, potting mix used if container. fertilizer used and how often, age of plant, how often do you water, variety, etc.etc.

Dave

What he said!