I wonder if this problem has been discussed and if it would be feasible to add a warning message that appears when a student clicks on Next without submitting.
This year, I had at least one student each semester who claimed that she had done her homework but she has no credit for it. I am almost certain that the problem was that after entering her answers, she clicked on "Next" instead of submitting it (in fact, I have caught myself doing that recently).
I'd bet she was doing it after the deadline. I don't understand how students would click on next without submitting, since they wouldn't see the green or red telling them right or wrong. But if they hit "check answer", which becomes available after the deadline has passed, then they'd see the green and also not have any credit for doing the problem.
I had one student act upset about that at the beginning of this semester. I guess maybe WebAssign acts differently, and/or their previous professor had set it up allowing late homework with a small penalty, so the student initially thought he was getting credit even though he was late. I made sure he knew that the deadline was the deadline, and no one else has ever commented on it.