DMZ dhcp release

[WildBoarCoffee]

Hi, I am running customer wifi on optional port, dmz.  We have been having problems with people not being able to connect to the wireless.  When I check the DMZ DHCP clients list it is showing about 120 people connected.  I don't think that it is releasing ip addresses.  I have the lease time set to 1 hour.  I have tried rebooting the firewall, however when I go right back to dhcp list they are all still there. There is no option to release any ip addresses.  I try to change the release time and apply, but this does nothing. 

The business is set up on the lan. the customer wifi is set up on optional port dmz. 

Any ideas? Thanks

I have this SA540 Security Appliance, and the firmware is up to date. 

[WildBoarCoffee]

I figured out that I can get the DHCP client list to refresh by going to DMZ config, then changing the ending ip adress down by one, from 10.1.20.200 to 10.1.20.199, then clicking apply. I then go back to DMZ DHCP clients and now the list is down from 120 ips to only a couple ip addresses.  Once this is fixed the wifi starts to work and people are able to connect without any problems.  I would like to know if there is a way to keep this from happening, or what is causing this.