In my work where I coordinate with people, it is hard when deliverables are not met at the time and day agreed upon. This is where flexibility is required and making up for lost time needs to be made as well.
Praying for patience and serenity has always helped me well. Perhaps because I have mellowed down for quite some now. When people miss their deadline due to other workload that interferes with mine, I try to move on after hearing them out and set another date. When nearly done, I strive to fill-in the gaps for as long as I'm capable of doing it.
When people disappoint us, remember that we are also human and sometime do the same to others even if we don't deliberately intend to. For as long as we are ready to listen and understand, we can work together to reach a compromise and help along the way.