All of us have problems sometimes with DHCP, especially if we use the Fortigate it as a DHCP relay server.
The following commands are used to best troubleshoot the DHCP process:
#diag debug en
#diag debug console timestamp en
The following is used if we use IPSec DHCP relay
#diag debug app dhcprelay 7
The following is used if we are using IPsec DHCP Server
#diag debug app dhcps 7