New User not getting IP from DHCP

Hi Team,

I face a strange issue in my DHCP environment. We have office all across the Globe and all DHCP leasing settings is set to 8 days. When ever a user from different location connected his laptop WIFI or LAN, the machine wont get IP. i did IP release and renew no luck. If i reboot my DHCP server, the IP gets assigned.

Please help 

Regards, Santhosh B S