Graeme, it's not only that you're an established author, but that your online personna in the GA forums oozes maturity, intellect, and respect. It's part of what someone mentioned earlier that an author's presence in the forums can affect whether a reader will read and/or review the story. If the author's known as brash, than the reader may be apprehensive about making a comment. For example, before I joined the tech team, I would never have commented on one of Myr's stories, because he comes across as scary and unapproachable (which I've seen learned that he's not).
But again, I seem to be shifting blame... I don't mean to. I guess I don't know how to convince readers to leave a review. Partly, making the process easy to do is extremely important. Another part is having the feedback plea at the end of the chapter. This provides readers with guidelines for how the author wants to be approached (email, PM, forum) and what kind of feedback is welcome (criticism, "I've been here" notes).
As I mentioned earlier the reader needs to be logged in to leave a review everywhere on GA. It makes it harder to leave anonymous feedback. Had anyone noticed a decrease in feedback with the implementation of spam filters on eFic? Maybe it doesn't matter.