Multiple possibliities here. Why do you assume wireless is not working before logon? When you say it works afterward do you mean you can ping machines inside or do mean you get the Internet? What security settings on wireless? More details.
Simply pinging machines inside does not mean DNS is configured on mobile machines. Without DNS they can’t find DC for logon. This could occur if you assume DHCP will fill in DNS parameters etc but in fact the mobile machines are falling back to your alternate network configuration which has only a good IP. People some times follow the Security manual for wireless which says “don’t allow DHCP via wireless” but forget to manually supply DNS and other IP parameters manually on the alternative configuration.