I see a possible different problem with your student. I think you've assumed that the student knows physically how to form dynamics. No. I'd bet your "bang, bang, clank, clank" student is using a separate initiation of power for each note. I see a possibility of playing five notes (in one hand) with only one initiation of power, and by this motion there will be some dynamics, and the student can modify the motion from there to get further degrees of dynamic contrast.
Also, you say your students don't remember. Here's an obvious question: do they have a notebook? Is there somewhere every week where you write down what piece they are to play, what parts, and how to practice it? If not, this could be a clue.
We've talked a few times about the arch. I've seen three techniques for teaching arch that don't require a pointless verbal explanation. First, from C. C. Chang, close the hand into a tight fist, then extend the fingers all the way, and finally relax. This should leave them in the natural arch for playing piano. Second, from Abby Whiteside, is to have the student imagine holding a delicate object like a baby bird or a flower in the hand, and then turn it over. Third, from Barbara Lister-Sink, is to use an object like a ball or orange and have the kid mold their hand to it. None of these require boring explanations, and, if you use the latter two, you may simply say "baby bird" or "orange" to have them form the correct hand arch in the middle of a piece.
anna crusis mentioned videotaping in a negative way. Why not in a positive way? Why not, when pieces are "finished" the kids record them, either to video tape or CD? That will surely give the student a sense of accomplishment, and also an easy way to see their progress over the course of lessons. Of course, you can't do this on every piece, since there are so many little ones in the beginning. Also, if the student has already been introduced to videotaping, when the student becomes more advanced the teacher has the opportunity to use videotaping as a tool to teach the student self-analysis and problem-solving from a new angle.
Perhaps part of the reason we have problems is students can't make the connection between the boring methods we use and the beautiful music made in performances by pianists. I'd bet many teachers can't see the connection either. Would you practice if you couldn't see how you were making progress, or how the stuff you were doing mattered when you want to play better pieces (in the beginning this is more likely to be show themes and music on the radio, I'd guess)?