Hi Jeff,
Bacula will not use any volume before retention perdidos. Unless you specify in your pool configuration "purge oldest volume". This directive does not respect the retention perdiods and will find the oldest used volume to recycle. It is recommended the use of "recycle oldest volume" instead. If none of this directives are specified, Bacula will stuck waiting you to mount a new volume that it can use for the job.
When I make this kind of decisions, I plan carefully the amount of space of each backup job I have and always there will be a volume to recycle.
Regards,
Ana