When multiple VLANs are configured on the Access Point (AP), clients on one of the VLANs might not receive IP addresses through Dynamic Host Configuration Protocol (DHCP).
DHCP service must be set up for each separate VLAN configured on the AP. This can be either the Cisco IOS DHCP server feature or DHCP service set up elsewhere in the network.
DHCP server capability can be set up on the Cisco Aironet 1100 and 1200 series APs running Cisco IOS Sotftware version 12.2(8)JA or later.
For an overview of this issue, refer to these documents: