Are you pruning it at all?

I think it wants to live in the garden, they make big shrubs....or as Cammomile hints have you been pruning the flower stalks?

What kind is it - changes how you prune.

As S says, it may be that the pruning needs tweaking. It might be too fierce. One suggestion - as it's healthy, and growing well, perhaps try pruning it more lightly so that the new growth matures a little earlier and flowers. First, however, it's possible that it will flower this year, there's plenty of time and my Hortensia types aren't showing bud yet, but they will.