Basically reCAPTCHA will fail to verify when certain users attempt solve the puzzles through their Google accounts (it's an issue which affected me for almost a year now).
I'm still not sure what you mean by verify. If you mean it just keeps failing to solve then that's Google's way of telling you that you do too many of them. I had a similar problem but after I stopped using sites with reCaptcha for a few days it was OK. Now I restrict myself to 5 a day and I just only have to check the box and don't even get a puzzle.