I have no experience with bonzai winter jasmine but there are a couple of thoughts which spring to mind. Firstly is to establish exactly what the plant is. Is this Jasminum nudiflorum? If so, my second thought is to wonder how an enormous sprawling shrub capable of covering many square yards can be kept as a bonzai for very long. I have seen these on offer but I think that keeping them dwarf AND getting flowers is bound to be difficult. And that leads me to the third point. In order to keep the plant small has it been pruned? If so it is possible that the flower buds have been cut off. Winter jasmine should be pruned immediately after flowering if it is pruned at all. Keeping it clipped back will almost certainly result in loss of flowers unless done very skillfully at the right time. Do you think that is what has happened to yours?

Well, it is a Jasminum Nudiflorum, that I know. And yes, I do prune it back in the spring and that might be where the problem lies.

Although, I do know that as a specie it is actually quite popular for bonsai training. I have seen many samples of mature specimens that, although pruned hard, still flower. You might be on to something though if the key is precisely where and when to prune.