Re: Student ability
Is there any physical skill, inside or outside of aikido, that this student *does* have nailed down? If so, can you two work out why that one is different?
A medical problem doesn't sound impossible. There's a description in Oliver Sacks' _The Man Who Mistook His Wife for a Hat_ of a woman who lost her sense of proprioception (body position sense) and had to relearn to sit, stand, etc. with great difficulty. A more subtle problem with proprioception, or the inner ear balance organ, or something interfering with the signals from brain to muscle, could really play havok with a person's aikido. If it's financially possible, a checkup by a specialist might be something to try.