I use the example here :
https://github.com/jackaudio/example-clients/blob/master/simple_client.c
I am only able to hear playback sound from my left speaker. I tried it on
two different computers with Ubuntu 20.04 LTS. What might be causing the
problem? Have you tried it on other OS's other than Ubuntu?

Thanks,

This example is a mono example. To get stereo processing you'll need to add additional input and output ports.