On the Akismet side, a few things to note. First, in wp-admin under ‘Plugins -> Akismet Configuration’ is everything green? You mentioned that you confirmed the API key is correct. Did you check the ‘Server Connectivity’ section? All of the IP addresses listed should be green. If not then you server isn’t able to make the needed requests to akismet.com to determine if a comment is spam or not.
The second, as Nakodari noted, make sure that you mark any comments that Akismet missed as spam. This allows Akismet to learn. If spam comments have been missed, or marked by some other plugin as legit then you may be throwing off Akismet’s results for your site.
Third, are there any other plugins on the site that could be interfering with Akismet? You mentioned WP-reCAPTCHA. Have you confirmed that WP-reCAPTCHA and/or any other plugins on the site are not preventing Akismet from doing its job?
As always you are welcome to drop us a line – http://akismet.com/contact/ – for Akismet related items.