The most common causes for this behaviour are;
1) You have a job queued up before the current one. Make sure that there are no other jobs queued up other than the one you are testing (or order by "Scheduled At" and ensure they don't come first). If your computer is trying to install something in production, it will not simply exist autoamtion and reboot into production, even if you force it to automation. Agent update jobs can cause this... so check.
2) AS BBC said above... The next most likely is that the first task in the job you are running is not set to automation, or is set to a different automation than what you manually selected. Check the job and make sure these are configured and match i.e. a Job configured for an automation environment called "WinPE x86" autoamtion will not work if you boot "WinPE x64" automation or a DOS autoamtion, etc.
3) Another possibility... the computer has a duplicate DS account or has just generated one because of a primary key difference. Check at the top of DS for a "New Computer" that is based of the computer's Serial Number. If your primary keys are set to something like "Serial + MAC", ensure that there are no other computers in DS that share that information. Unless the primary key is unique, you could be picking up another DS account that has no tasks assigned.
4) there is probably a fourth... but I can't think of one at present.