Did you configured the ip helper on the Switch???
If you are configuring for example
In your switch core you have like 5 Interface vlan, your AP is on vlan 10 but the dhcp server is on vlan 15 you need to configure the ip helper on the switch, i mean that if you are trunking the vlan from the IAP to the swtich.
If you are plugging the IAP to a L2 switch you need to make sure that the VLAN which the IAP is can reach the swtich core which got the interface vlans, on the switch core configure the ip helper pointing to the dhcp server.
hope that helps
Cheers
Carlos