We use the slave Availability option "Take this slave on-line when in demand and off-line when idle" for many slaves and we found when we restart Jenkins (Tomcat stop/start) what we can find is the slaves where this settings is enabled, comes as offline and we have to launch all of them manually. Normally the slaves with take offline when idle settings, will come online when a job tied to the slave is triggered but in this case that wont happen.

Looking at the api/xml of such slaves and that are gone idle and got offline, the difference found is no <offlineCause/> tag in the issue slaves. Seems like the cause is not updated to slave properly when the Jenkins is restarted.