This error happens when:
1. Two NICs are required(one for monitoring only, another for blocking). If you're filtering 100+ computers, we recommend you to use two network cards. Please check this topic: http://blog.imfirewall.us/When+Shall+You+Use+Two+Network+Cards+For+Monitoring+And+Blocking.aspx
2. There is firewall policy applied on the blocking adapter. WFilter sends RST packets to terminate connections. However, inproper firewall rules can block WFilter packets from being sent. We recommend not to enable any personal firewall in the WFilter computer. This topic might be helpful:
Modify ESET personal firewall settings to make WFilter work.
3. If you're filtering multiple VLANs, please make sure the blocking adapter belongs to a VLAN which can communicate with other VLANs, otherwise it will reduce the blocking success rate.