Vijay,
Do the controller configuration in both WLCs are equals about parameters like WLAN id, ap groups and etc?
Because if the WLAN id are different you can lose the VLAN mapping when the AP migrate from one WLC to another.
About FlexConnect Group, it helps so much when you need to apply vlan mapping in the WLAN in FlexConnect APs, because FlexConnect group configuration you can maps the WLAN id in some VLAN and all APs that you add in this flexconnect group will get this configuration without the necessity to access each AP and map VLANs individually, because all APs inside this flexconnect group will get the VLAN information in that WLAN id.
But when you create the FlexConnectGroup is important to add the same group in both wlcs and add the APs mac address in both WLCs FlexConnect Group, because when one access point associates to WLC01 it receives a FlexConnectGroup configuration and when go to the WLC02 it needs to receive the same configuration.
http://www.cisco.com/c/en/us/td/docs/wireless/controller/7-6/configuration-guide/b_cg76/b_cg76_chapter_010001010.html#d34284e1680a1635
I hope it helps.
