Node child spawn and bash

I use a raspberry pi. During a node sockets server runs, (socket?on / emit) i use a child process who spawns a bash program who launches ffmpeg (video encoding -long task-)
Problem : the server is not responding during ffmpeg encods if i want to control the server with wifi. I have to waiy the end of encoding (on the machine raspberry pi, in localhost, all is good)
I have try workers, in witch i launch ffmpeg in js, same result.
Cause ; is the raspberry pi ? or ?
Merci beaucoup