I have no sure answer to share here, as it sounds like you are doing everything right to nurse a weak plant along, but discovered a fascinating post elsewhere with great photos of the process of flowering in this plant, and all of what’s going on. Worth a look:
Welcome! I’m Margaret Roach, a leading garden writer for 25 years—at ‘Martha Stewart Living,’ ‘Newsday,’ and in three books. I host a public-radio podcast; I also lecture, plus hold tours at my 2.3-acre Hudson Valley (NY) Zone 5B garden, and always say no to chemicals and yes to great plants.