user data not saved in session after login

i am facing a problem with my user registration and login script, register user working fine, after activation of user when try to login first time, providing proper username and password it redirects back to login page not going to user panel, if i login again it work fine, very first time user login not saving user data in session, i have debugged i can see user data in session before redirect to user panel but after redirecting nothing in it.