I have always had this same issue. Shown as signed on and my profile picture shows correctly. I could click as many stars on the review as I liked, but even after writing something, in the review, th? ok button remained greyed out so I could never send the review.
SOLUTION: The simple answer was - type something in the subject section - Easy answer, but I guess I never realised there was even a subject section. Even though it appears directly above where you write your review.
Hope this helps!