Answer: I don't think it's your effort that's failing, I think it's that you're trying to achieve the impossible. Some Yuccas are stemless and others can reach tree size. You're probably growing the shrub-type (Yucca filamentosa or Yucca glauca) which is not capable of producing a tall stem. Yucca gloriosa will grow 6'-8' high, often with a tuft of sword-like leaves at the top. So, outside of planting the variety that will grow tall, you won't be able to have a palm-like Yucca.
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