If jobs are added in two batches at different times, does the oldest batch have to be completely finished before the newer one is started?

My backups are made fd -> file (disk) store -> copy to tape (an old LTO2 drive).

One client is huge compared to the others, copying it to tape takes some time (about 5 tapes).

My problem is that new incremental backups that should start while it is being copied, just sit there “waiting for execution” until the copy operation has completed – yet they are set to run at a higher priority, neither the storage they are to be written to nor any source fd is in use at the time, and the sd and director both have sufficiently high ‘maximum concurrent jobs’ settings.

I’ve gone over all configuration files several times to see if I haven’t forgotten a ‘maximum concurrent’ or so, but I find no reason why those jobs shouldn’t start.

PS: sorry if this is a repeat question, it sounds rather familiar while I am writing it, but I didn’t find an older version.  It’s also possible that I started writing it a few months ago, but then decided not to post it and continue searching a bit more ;)