Fortigate session started but data not transferred

In FortiGate logs, I have found Some external Outbound traffic that happened from the internal network with device action-Start.As per Fortigate documentation I understood that Fortigate allowed the corresponding traffic and TCP session started. I have checked the bytes in and bytes out field values, which were 0, meaning that data was not transferred. So, the session started but data was not transferred in this communication. I would like to know whether FortiGate played any role in this data transfer that's why data was not transferred?