Once a template fails it is not accessible from the machines.
No machine can pick up a failed template only active templates.
As for locked templates. I have a single master template which is displayed as a shortcut in the Public folder ( i never place actual templates in Public but thats just me )
this is then chosen and locked as it creates the new build task.
The locked templates can be used again if the engineer started the build task again from their console. But not from provisioning login. They could also add another machine to the task and delete the one they were building. And yes it would use the locked template but it would just start again.
if provisioning is running smoothly and your template is good ( ie the failure reason was user error or a network break ) you should be ok using the same task again. UNLESS you change something in the original template the locked one was created from as no new changes will pick up. Hence the word locked.
If the template would not start then the core thinks the build task is still active. Was it definitely failed in the console?
You cannot run two provisioning jobs on one machine. The previous job has to fail for a new one to start.