port mirroring: Cannot capture all traffic from my virtual machines.

Hello! I'm working with 5 PC's: one for port mirroring and I want to capture traffic between the rest. In all the PC's I have virtual machines running (ubuntu, virtualbox). In PC 1, 2 and 3 I have just one VM, and in the PC-4 I have 3 VM running.

My problem is that I cannot capture all the traffic from the PC-4, I get only the traffic from one virtual machine and for the 2 other VM wireshark cannot see their traffic.

How can I solve this problem? Thank you!