Hi, what kind of spam? Of course, it cannot prevent human spammers. I get those passed, too. But, they are very rare. This is because recaptcha itself does not treat them as spammers (yet), so, they pass. The only way I would see it as a bug, is that they are marked as spammers by reCaptcha API return response, and the plugin passes them through. Otherwise, no.
In order to catch human spammers, that pass reCaptcha, you need to catch them later, either by using Akismet, or custom blacklisted keywords etc.
You have to understand that reCaptcha does not analyze posted content in any way, only looks for overall activity, IP address etc.