Mine flower every year but they need to feel seasonal changes in temperature and light to flower. Usually, they flower at the end of winter and then they grow new leaves and shed their old ones. I don't disturb the roots with repotting if I can avoid it and I also only give rainwater. I repot in winter, when they are not actively growing and should be kept mostly dry. I would not repot after flowering.
Bella is a subspecies of L. karasmontana http://www.llifle.com/Encyclopedia/SUCCULENTS/Family/Aizoaceae/12167/Lithops_bella
Thanks a lot for the advice and the link!