At first, I thought it's something to do with CustomGUI, but I added a "message" command and that doesn't trigger as well. No errors in console. What's the issue?

https://pastebin.com/C44bU57G

Just tested it and your items.yml seems to be working just fine as you can see;

Are you sure you don't have any plugins that are preventing certain events from triggering such as PlayerInteractEvents? Also you are left clicking correct? Because those are the actions you specified for the commands, you never added a right click or right click block to command actions.

That's odd. I'll test more and come back to you

Its been a while since I have heard from you, are you still having this issue?

EDIT: If you happen to come back to the ticket feel free to reopen it.