I found this response to “Why Don't My Hyacinths Bloom” on another forum, it might help.
Mine never bloom either, and I think I know why. I fertilize my hyacinths by adding potash, nitrates, and occasionally iron to the pond water. They grow very well. They are big, green, lush plants, but they don't bloom. I keep them to remove phosphates from the pond water, and they do a wonderful job of that. The pond water is phosphate depleted, and algae free.
And that's the rub. Terrestrial gardeners know that adding excess nitrogen and minimal phosphate produces plants that grow well, but do not flower well. To promote flowering they cut back on the nitrogen, and add plenty of phosphate.
So, I think that is what happens to my hyacinths. It might be what is happening to your hyacinths. I don't know what nutrients are plentiful in your pond, so it is just conjecture.
Should you add phosphates to your pond to promote blooming? It would be a great experiment. I would love to see someone else perform this experiment. It might cause the hyacinths to bloom. It might also cause an algae bloom. There might be a certain amount of phosphate that would cause the hyacinths to bloom, but not the algae. I'm not going to do this experiment in my pond, because I hate algae.
If anyone takes the phosphate challenge I would love to hear all the details. Horton added some all purpose fertilizer to his pond once, and the algae didn't bloom, but I'm not sure he added enough to see a difference. I didn't hear if it made a difference to any hyacinths.
It would be hard to know how much phosphate to add. If you try to solve this, remember, it is an experiment. You don't know what will happen. Keep good records.
I suspect that this might be my problem as well because my hycinth are tall, dark green, and lush. I do not know if I will try this either, I hate algae. If anyone is willing to take the plunge, let us know.