It just occurred to me as I am reading old process literature, that it was quite easy to skip over procrastination based on certain approaches to studying process.
A certain segment of process researchers were looking for a scientific way to study writing. They used tape recorders and coding systems. They wanted to be objective and to have replicable results. These are essentially lab-based studies, then. The researchers would set up the lab, give the student an assignment, and study them on the spot, likely asking the student to talk through their thoughts and actions as they wrote.
This kind of research is useful and even important, but by being lab-based, it loses the ability to study procrastination at all. I mean, I suppose some students would still delay in a lab environment, but it's a rather different kind of situation than being given an assignment to turn in within a week, let alone by the end of the semester.
This makes some sense, in that the researchers were more interested in writing itself, not "doing homework." On the other hand, psychology researchers have tended to be more interested in the "doing homework" side, and this explains why I have found more of their research to be directly relevant to my study.