Google Recaptcha not showing up after postback

I have a problem regarding displaying google recaptcha after postback in asp.net web form application.when the page is loaded for the first time,recaptcha works fine for my registration page,but i am checking for enter username in the submit button click event,so i need to preform postback,and if the enter username is already available in the database then again i need to keep user to the same registration page.Note that i am not reloading my registration page.Any help will be really appreciated.

<recaptcha:RecaptchaControl ID="recaptcha" runat="server" PublicKey="myPublicKey" Theme="clean" PrivateKey="myPrivateKey" />