Two possibilities:
1. Misconfigured port mirroring.
1). The port connected with the router/firewall shall be the monitored port.
2). The port connected with the WFilter shall be the monitoring port.
3). Both inbound and outbound traffic shall be mirrored.
4). The ports configured shall be consistent with the reality cable connections.
Please follow this guide to check whether port mirroring is properly configured:
How to check whether port mirroring settings are correct?
2. Firewall program in the WFilter computer. We found some firewall programs(eg: "Nod32, Comodo") will block non-local packets. So packets of other computers can not reach WFilter. So please also shutdown your firewall program in WFilter computer to make a try.