Did you specify the capture point and direction properly? By default, pktcap-uw captures only ingress traffic. If you capture bi-directional traffic, you must exec pktcap-uw with --dir 2 option.
Capture packets according to the direction of the flow with regard to the virtual switch.0 stands for incoming traffic, 1 for outgoing traffic, and 2 for bidirectional traffic.
By default, the pktcap-uw utility captures ingress traffic.
To determine the capture point, the IOChain diagram in the following blog post is helpful.