Wither Skeleton Farm not working (Low spawn rates)

Hey everyone, I'm having trouble with a farm and could use some advice.

I built [Voltrox's Wither Skeleton Farm](https://www.youtube.com/watch?v=TMKrBswfSgM&utm_source=gemini), but it’s barely producing anything (yielding around 6 wither skeletons total). I followed the tutorial block-by-block, though I noticed a few YouTube commenters are having the same issue.

**Context:**

* Playing in Singleplayer, version 26.2. * I've already checked the surrounding area for hidden parts of the Nether Fortress. There are no fortress sections taller than the platform I measured out.