Hi Ken,
I show how on the first video on this page, from about 4:30:
https://www.brendan-power.com/harmonicas-irish.php
After the valves are removed, reduce the relevant blow reed gaps and the draw reed response should improve significantly. If it doesn't, it indicates that you have sir loss elsewhere, probably in the slide/mouthpiece assembly.
Half-valving only works well on good quality airtight chromatics. What model are you using?