Question

If you apply, say, 50% quantize once it does what you'd expect. If you apply the same 50% quantize to the same notes again, nothing changes. It would be nice if you could compound quantizing.

Answer 1

Works fine here. What do you do so that it fails?

Answer 2

I don't know? I just set the quantize start to 50%, apply it, the notes move closer to the grid; it works. Then I apply it again without changing the selection or any settings and the notes don't move again. Not unless I change the settings to something higher eg. 60%.
Am I missing something?