Have you tried using the Fortinet connector(s)?
Can we get more info about your setup? How did you end up with the WS_FTP connector - was it automatically selected? As byrona mentioned, I'd expect the Fortinet connectors to be a better match, but maybe the automatic process somehow selected the wrong ones? You can manually configure connectors if for some reason the default choice was incorrect.
video on scanning for new nodes: [VIDEO] How to Scan for New Nodes with SolarWinds Log & Event Manager
or, video on using the add node wizard for syslog nodes: [VIDEO] How to Configure Syslog Nodes in SolarWinds Log & Event Manager
or, video with multiple methods, including manual connector configuration: [VIDEO] Adding Nodes, Devices and Systems with Log & Event Manager