Re: Student ability
One of the interesting challenges that I discovered when I started to teach was choosing what words to use with what students to draw upon their existing background, knowledge and ability. Some words help some students understand a concept amazingly but don't do a thing for someone else.
As an example its hard to tell someone that the reason we keep moving our hands and body in the same direction as the throw even after uke has fallen is for follow through and commitment, as in finishing a golf swing, when that student has never played golf. You have to find another analogy that will "click" with them.
Or some students plateau for a really long time, and then make huge strides.
It sounds like your case is more difficult than many, but finding out the background of the student and more about what they are good at may help. I sometimes remember things that my instructors said to me that didn't really help (because of my background) and try them out...amazingly enough that sometimes works. Lynn's suggestion of a private session is, I believe, a good one. You also might consider using video as a training tool.