I've been waiting all summer for a bloom on either of the two hibiscuses I overwintered last winter. I haven't got a single flower. I did get a bud, and I was all excited. Then it yellowed and fell off before ever blooming. Drat!

What's the likely reason? The foliage looks good to me, green and glossy, and they've gotten a lot of new leaves since early spring. I keep them well watered and feed 'em every other month or so with Carl Pool Hibiscus Food. Last summer they did pretty well. Why no flowers this season?