Yucca in the wild have a big/deep tap root. They also grow with more direct sun than any of us could survive. If I were you, I would unpot it and check the root. Is it firm and healthy? If there is rot, to what extent? Is it straight, coiled around at the bottom (as if it wants to grow deeper)? Any sign of bugs around the growth point or the small side roots? Some parasites are hard to see at first glance. If it's gooey when you squish it, no matter what it looked like first, tell us about it. My mother grew some yucca outside at her house and one year decided to transplant them. She couldn't/didn't want to bother, getting to the bottom of the root, so she cut it off at about a foot and a half and relocated the tops (these were mature plants
). It took them a year or more to recover, but then they grew as well as they had before, blooming and all. 2 years later she found shoots coming up at the original site that eventually developed into reasonable plants
. The moral of this story is ... yucca are forgiving. You haven't necessarilly lost yours yet. Be of good cheer,check it out, and tell us what you find!