ERROR: Unable to communicate with master! when I'm using a launch with <machine> tag

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Hello, I'm trying to establish a master/slave configuration by using 2 PCs, the Master has launch file in it that starts a node, a rosserial_python node for Arduino, one the Slave machine. I'm able to see that the node works properly since that I get an error when I disconnect my Arduino, but the problem is that when I try to list the currently active topics whether I do that on the master or the slave machine I get "Unable to communicate with master!" error. but if I ran any other launch file or just the roscore on the master I'm able to use the rostopic list command, and when I run roscore on the slave machine I get "roscore cannot run as another roscore/master is already running." so it's really confusing, can you help me?