Yucca, will it flower in a container??

by Kathi
(Western NY)

I have 2 Yucca plants which I have planted, out in the full sun, in 2 VERY large (15inch square) containers, marking the ends of our driveway near the road, but they have not flowered and this is their 3rd year. Will they not flower because of being planted in a container? Is there some special fertilization necessary for Yucca to flower? I do also have 2 more Yuccas in our yard, also in containers, with some partial shade, that have also not flowered. So I think the containment may be the problem.