WX GUI not responsive and getting grayed out

I run a flow graph under GRC with WX GUI widgets (sliders, FFT sink,
etc) on
Linux. The graph works and I can receive signals, but the GUI is not
responsive (I can’t move a slider, choose between radio buttons), and it
gets grayed out after a few seconds. The flow graph continues to work
the graphical FFT sink continues to work: when there is a signal, I can
it on the FFT sink and I can hear it in the audio sink.

If I generate a python script from the graph and run it on the command
I get exactly the same behavior.

Why is the GUI not responsive?

Thanks, Sivan T.