I'm don't know about alpina (I don't have that one yet). But I've have a fair amount of success with other utrics. The two things that I've found to work for me are (a) very high light and (b) fluctuating water-levels. Without high light, they don't seem to flower. However, the fluctuating water levels seem to trigger flowering once they have sufficient light. I've heard some people say that they have to be pot-bound to flower, but I've never found that to be an important factor.