Other people’s schedules – lp

I understand that other people’s schedules aren’t something I can control. But what do you do when your own schedule is determined by someone else’s schedule? Like when I’m waiting for something from someone and they don’t get it to me or they don’t follow along with the timeline that was set out. Particularly when that person is the boss or a client. I feel like I’m putting out fires and in a very reactive mode.