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:
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.