Hi @sunnyg,
How frequently are you watering it?
Have you increased the size of its pot since you purchased it?
I suspect it is either due to overwatering or it has outgrown its pot.
Based on what I am seeing, I'd suggest you repot it in a slightly larger pot with some fresh, high-quality potting mix, such as this Scotts Osmocote Premium Plus Superior Potting Mix, and you really focus on getting the right watering schedule.
Most plants, Nasturtiums included, don't like to be wet constantly. Instead, they prefer a good drench once or twice a week, depending on their soil conditions. When watering, you'll want to check that the soil no longer has moisture in it before giving them another drink. You can do this by pushing your finger into the top 5cm or so of soil. If it is still wet, wait a day or two; if it is dry, you can go ahead and water.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
