Hello there!
My situation: I found the template "Book a new calendar event after a new SharePoint List item is created" and used it as reference. It uses the "When an item is created" trigger and works perfectly. I also used the "When an item is deleted" trigger and that one also worked. But when I use the "When an item is created or modified" trigger, it only runs when I start it twice.
e.g.
I have six items in my list. Then I modify one of them (change some values). After that the flow doesn't get triggered. But when I modify a second item in my list, for some strange reason, the first flow gets triggered and then the second one. It follows that pattern: First one doesn't trigger, second one triggers both.

I found several posts which are about the "created and modified" trigger, but no one wrote about what I experience. Any clues why it won't trigger the first time?