nethogs shows 0 KB although I have traffic

I am running a network related program that performs TLS handshakes with many hosts. I ant to monitor the network traffic so I used nethogs. It shows 0 KB.

NetHogs version 0.8.5-2

PID USER     PROGRAM                    DEV        SENT      RECEIVED       
  ? root     unknown TCP                            0.000       0.000 KB/sec

  TOTAL                                                 0.000       0.000 KB/sec

I have previously run this program and nethogs was showing me information about the network traffic. What is the problem? How to see the ongoing/outgoing traffic? Are there alternative tools?

I use Ubuntu 18.04. The command I entered:

sudo nethogs

I also tried with the adapter name that I obtain using ifconfig:

sudo nethogs enx9cebe8b0d86a