You have a build running, and it only gets a maximum of 30 agents, even though eMake says it could use more, and there are more free agents in the system.
Check the “Max Agents” setting for the build class you are using (usually that will be the default build class). That is set to 30 agents by default because most builds start to show degrading performance at around that number of agents. This is very much dependent on the kind of build you are running, so you may want to experiment with that number.
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OS versions: All