Recaptcha involves a call to Google run servers (recaptcha is Googles own anti spam captcha)
They ban IP's and entire blocks of IP's if they think they are being automated.
Pligg and some forum platforms have recaptcha on about every site, so if you are linking from hundreds/thousaands of them - this can get you IP bans very quickly.
Good news - ban is normally temporary
Bad news - You need a lot of proxies to run Xrumer on these sites with different B and C class preferably.
Some sites hardly need proxies at all, but others really do need loads.
My Xrumer servers have 200 each, bought from 5 different providers to make sure the subnets are suitably diverse.
Scritty