You have not given us much to work on.
However you have not mentioned if your vlan's are using statically defined IP's ? DHCP ?
Is DHCP server giving the default gateway of the fortinet ? if they are static have you configured the fortinet as default gateway ?
On your switch just make sure your trunk ports are set to allow all vlan's, then test.
You can play with locking down specific vlan's on specific trunk ports later.
fortinet ------- trunk ----- switch port.