Hopefully you're not simply going by that message to determine that you have no Internet (Windows lies sometimes). Anyway, make sure that reading top-down you have a firewall rule that allows traffic from the subnet or address of the device in question to the Internet, and that this rule is above any rules that might block the traffic. That's just firewall 101.
You didn't mention either FortiAP or FortiWiFi. So I'm assuming it's a third party AP connected to your FG100D. Since you said "some of laptops", I assume other WiFi devices have internet when it's connected to the same SSID. Then what you need to do first is to sniff traffice at the FGT with the source IP those packets are supposed to be with. Depending on how your AP is configured It might be NATed, which is unlikely if other working devices are using the same SSID, or without NAT and the source IP is the laptop's IP.
If it's hitting the FGT, then you need to proceed to the troubleshooting at the FGT like Daniel suggested. But if it's not hitting, then problem is at the AP.
