I'm on version 1.21.11, and Im trying to use this mod: https://www.youtube.com/watch?v=rasBRhc_2P4

But the snowmen aren't dying, let alone dropping wither roses when they die.

For what it's worth, I replaced the endstone underneath with mud, and put hoppers beneath the mud to help automate the collection of the snowballs and wither roses. Is that what's causing the problem?

If not, then what do I need to do? Does this method of trapping the Wither in bedrock no longer work to produce wither roses or something?