Certified GKH Gardening Expert
It most likely isn't getting enough light. That is the biggest reason plants don't bloom indoors. Begonias need bright but indirect light such as from a south facing window. They also need humidity such as from a pebble tray filled with water. A fluorescent or grow light can help if window light is not available.
The next thing could be lack of nutrients. How long has it been in a container? It may need the soil replenished or a slightly larger container. Here are their growing needs you can compare with yours: