VSTS continuous integration triggers not working

Adding an answer so this more visible. @starain-MSFT comment was the solution.

I added a new build with the same trigger and it fixed the other one not getting triggered. Then deleted the new build I had added and everything is working.

Cloning my build definition didn't work, unfortunately, and neither did saving a trivial change to the build definition. Instead, I had to explicitly disable the CI trigger, save, then re-enable it and save it again. (Annoying, because I had to redefine all the rules.) But give it a try if the accepted answer doesn't do it for you.

I also wonder if adding a random branch to the triggering branches list, saving, then removing the random branch and saving again would work (and save you from having to redefine everything).