They lack procedures or heuristics to help them reorder lines of reasoning or ask questions about their purposes and readers.
What's funny about this statement is that it makes it seem like students need "procedures" or "heuristics" whenever they're writing so that they can achieve writing in cohesion, and this kind of conflicts with what Mike Rose found in “Rigid Rules, Inflexible Plans, and the Stifling of Language.” While Nancy Sommers says students don't have the "heuristics" to get a complete understanding of the larger issue in their writing, Mike Rose gives examples of students who struggle because of the heuristics they follow to do such. This makes me feel kind of conflicted as to how I should write in my projects, but I think a good compromise would be to first write drafts that don't follow rigid rules that would restrict me in my ideas, and then to consider certain "heuristics" to helps me understand what larger issues are at hand.