Go to configuration> Wireless> AP Configuraton. Edit the ap-group that your access points are in.
Expand Wireless Vlan> Expand Virtual AP. Click on the Virtual AP that corresponds to your wireless network. In the right pane you should see "Drop Broadcast and Multicast". You also need to enable "Convert Broadcast ARP requests to unicast", as well.
Click Apply in the lower right hand corner.
See if that makes a difference.
If it does, for best performance, you should enable this on all your Virtual APs in that AP group.