Launching a slave Jenkins node when a change is pushed to GitHub - github

Question

I'm trying to build a project on Jenkins when a change is pushed to GitHub.
I'm using GitHub plugin and building on a slave node.
If the slave is online, everything works fine. However, if the slave is offline, Jenkins doesn't try to launch the slave node and ignores the notification from GitHub.
The following log is output to jenkins.log when the slave is online:
May 30, 2016 4:16:31 PM org.jenkinsci.plugins.github.webhook.subscriber.DefaultPushGHEventSubscriber onEvent
INFO: Received POST for https://github.com/myname/myproject
May 30, 2016 4:16:31 PM org.jenkinsci.plugins.github.webhook.subscriber.DefaultPushGHEventSubscriber$1 run
INFO: Poked MyProject
May 30, 2016 4:16:33 PM com.cloudbees.jenkins.GitHubPushTrigger$1 run
INFO: SCM changes detected in MyProject. Triggering #22
However, if the slave is offline, the build is not triggered:
May 30, 2016 4:15:58 PM org.jenkinsci.plugins.github.webhook.subscriber.DefaultPushGHEventSubscriber onEvent
INFO: Received POST for https://github.com/myname/myproject
May 30, 2016 4:15:58 PM org.jenkinsci.plugins.github.webhook.subscriber.DefaultPushGHEventSubscriber$1 run
INFO: Poked MyProject
Starting a build by clicking "Build Now" always makes the slave node online. How can I build a project by GitHub changes on a slave node which might be offline?
Update:
I found the following message on "GitHub Hook Log" of the project:
Started on May 30, 2016 6:00:46 PM
We need to schedule a new build to get a workspace, but deferring 561ms in the hope that one will become available soon (all_suitable_nodes_are_offline)
Done. Took 0.23 sec
No changes
Update(2016/06/01):
This is the current slave setting:
Contents of /var/lib/jenkins/bin/start-slave:
#!/bin/bash -eux
gcloud compute instances start ci-slave --zone us-central1-f
ssh ci-slave /var/lib/jenkins/bin/start