The Jenkins EC2 plugin no longer launches stopped nodes. Unfortunately I'm not sure exactly when it stopped working - I wasn't sure that was the issue until later, due to unrelated issues caused by too many nodes spawning and having to be killed.

If I use Manage Jenkins -> Manage Nodes to start a stopped EC2 node that a build is waiting on manually, the build proceeds.

Builds succeed when the EC2 plugin spawns a new node for the first time. It's only a problem if the node is stopped for idleness - the plugin doesn't seem to restart it.

Builds get stuck with output like:

Triggering bdr_linux ? x64,debian7
Triggering bdr_linux ? x86,amazonlinux201209
Triggering bdr_linux ? x86,debian6
Triggering bdr_linux ? x64,amazonlinux201209
Configuration bdr_linux ? x86,amazonlinux201209 is still in the queue: Amazon Linux 2012.09 EBS 32-bit  (i-b848fbfa) is offline
Configuration bdr_linux ? x86,amazonlinux201209 is still in the queue: All nodes of label ?amazonlinux201209&&x86? are offline

where there's at least one node with that label stopped, ready to start and use.

There's no sign that any attempt is made to start the node.