are you using TOR or any other IP masking solution?
The more reCaptchas get solved or requested by a particular IP the harder they become, so if you are behind a IP that has been used for solving/requesting a lot of reCaptchas it is close to impossible to get a readable one (took me about 80 tries till I got one right when I used TOR). I don't know what exactly determines this difficulty but it is definitely there and Google talks about it here: http://googleonlinesecurity.blogspot.de/2013/10/recaptcha-ju...
