After careful analysis - I've sorted it out - and it was my fault. For some of the tests we had the "Marks" check box unticked in the the "Later, while the quiz is still open". So having this unticked means the relevant grade book entry is hidden (ie greyed out)
We still have the problem - which I reported in another posting - that the order of displayed quizzes in the grade book does not match the order we have in the course. Is anyone else seeing this?