Jobs sometimes not enqueued

We have seen a number of cases and had reports where builds have been triggered but they don't appear in the queue on Bamboo's "Build Activity" page.

When viewing the build, the jobs claim to be in progress but have a status of "Job has not yet been queued. Waiting for prior stages to complete." However, no prior stages are queued or building. Additionally, many agents will be sitting idle even though this build is waiting to be run (probably because it hasn't "really" been queued).

Eventually the build will go grey with a status of "Reason for not building is unknown".