This one eluded me for several years. I have always been intrigued by the Process Builder option “Do you want to execute the actions only when specified changes are made to the record”. It’s pop-up help and its complete help page never enlightened me much. But today I really had to understand this concept, because I had a problem that was directly related to it.
The process on which I am working has several conditions (with actions of course) and the actions and conditions are dependent on each other. So the order of the conditions in the process is important, but I had figured that one out. The first condition checks whether some boolean is true, with the option “Do you want to execute the actions only when specified changes are made to the record” set. When I test this, it looks like only this first condition is taken into account. And its actions are executed only once, and never after that first time.