Hmmm, your post pointed me to the possible cause of the problem.
To answer your question, I'm mirroring port 11 (monitored port) to port 14 (monitoring port), so it's only one port that I'm monitoring.
Yes, I can do ALL ports, but I'm afraid it's not going to fix the problem because this is not core switch; it's just gateway switch for one of our subnets.
Is it correct to assume that the appliances have to be connected to the core switch of the network, and all ports of the core switch to be monitored by the SPAN port?