Although I am member of a grading committee, I strongly believe that aikido would be better without any grades. They become instruments of power and oppression, they involve a lot of money, and so on.
But we have them, so we should try to do it right. I want everybody to take their grades, otherwise only those who want them will do so, and the attitude in the aikido community will change for the worse.
Testing should be a routine thing, at regular intervals where those who have done the time get invited - or apply and try, without invitation.
If a teacher consistently refuses a student to test for the next grade, without giving good reason for it, then the student needs to find another teacher. That's really what the teacher is saying, even if unknowingly, with such an attitude.