Hi Srinivas,
If you have a DHCP server that is running on the same VLAN which is used for users of PSK-enabled and captive portal-enabled SSID, it should be working as a charm.
Please take care if your captive portal SSID bounds the user traffic to this appropriate VLAN.
If the VLAN ID and tagging is appropriate, do you implement role-based access control? Perhaps guest role for captive portal SSID does not allow for DHCP communication.
As Andrew said, posting the config would make it much easier to help you with troubleshooting.
Regards,
Tomasz