I have the same problem (for Mac and iPhone apps) and contacted Apple about this.
The English speaking support said it is a known error and that they are working on a solution. The German support however (which I contacted first, unfortunately) gave me a refund of the apps I wrote them as an example. They told me I should buy them again – which is not possible, since the MacAppStore then gives the message "You have already purchased this app..." Ironically, for one of the two apps I was finally able to write a review, even at this point it wasn't listed in the purchased apps list anymore.
I've never asked for a refund, and it's causing me more trouble than I had before: now I cannot update these refunded apps anymore (since they are not listed in the "Purchased" list). So I have to check manually if there is a new update (comparing version numbers) and then delete the app and "purchase" it again. Of course I would simply buy the apps from the developer directly, however, they are MacAppStore only.
When I asked the AppStore support about a solution for this problem, they said it's technically not possible to write reviews for refunded apps (? yes, of course they were not really reading what I wrote them. I guess support at Apple is done by Siri now) and gave 10 free iTunes songs.
I think reviews are very important for both customers and developers and it's not just a minor inconvenience, but a serious problem; what is especially disturbing, is that it's not the same for all apps: for Pixelmator, I had no problems to post a review, for Jumsoft's "Relationship" it doesn't work, for some apps it worked immediately, for some after a few weeks, for some never.