why has my buganvillia stopped flowering?

You don't say if you're growing it indoor or outdoor - if outside, the cooler nights might have affected it. The other thing is, what are you feeding it with? When you repotted, the compost would have a general purpose nitrogen based feed mixed in with it - during the summer, these plants need a higher potash content, so something which encourages flowering, like tomato food, would have been appropriate. However, they usually only flower during "summer" so that I wouldn't suggest you use tomato food now, as its so late in the season. Feed it with a general purpose feed next spring, then switch to a higher potash one until end July.