Water lilies are fairly large plants that need room to spread their leaves out on the water's surface. Even the dwarf water lilies can be quite large. Mine end up covering an area about 3ft. by 3ft. and they do bloom with more than one flower once they spread out.
They also require several hours per day of direct sunlight and some fertilizer. So if you are meeting its needs as far as sunlight, and nutrients are concerned, then I'd say you need a bigger container for it so it can spread itself out wider. The more leaves, the more sunlight it can gather and convert to food and energy to bloom and reproduce itself. Here in the US they sell little round "pools" about a meter across for small children and dogs. If I wanted to grow a water lily on a balcony, that is what I'd plant it in.