Not both transmitting voice, but just the client that was started last

Does any possible explanation for this behavior come to mind?

- I have 2 clients to test -- whichever I start first will be able to hear the other, but the other won't be able to transmit to the first
- when I then stop the Unity client who started, then run it again, finally both will transmit their audio (auto disconnect is set to off)

Thanks!