There might be a number of reasons for this. Your plant may only be a single flowering variety and once it has flowered, it won’t flower again until next season. These plants typically are the more showy, taller and more spectacular plants that are great for centrepiece plants in a container. If your plant is a repeat flowering variety, it could be that it needs feeding. A liquid tomato feed will keep the flowers coming through it’s flowering season. If your plant has lots of spent flowers or has developed seed head, you should remove these to encourage further flowers.