Forms Authentication Cookie Not Persisting between Postbacks

I have the follow code in my button click event
//Sets the authentication cookie FormsAuthentication.SetAuthCookie(Text_txtUserName.Text, true);
It looged me in and I'm authenticated but when I do any kind of postback. I get logged out!
Here is my web.config line.
<authentication mode="Forms">
      <forms name=".ASPXAUTH" loginUrl="default.aspx" timeout="10" slidingExpiration="true" protection="All" path="/" cookieless="UseCookies" />
    </authentication>
 
What am I doing wrong!!????
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