Submitting considerably more than 20 tasks at once causes a rejection error for some tasks. These tasks do not run, instead they hang in a "Waiting" state. You can see an error in the logs reported from quartz:
To reproduce, you can run the test here: https://github.com/sonatype/nexus-internal/pull/1671/files#diff-f47ab1796bb11e077d08f418a8fcf42e
Changing the maximum tasks to 50 instead of 21.
Also, you can use this groovy script to schedule a large number of tasks to run at once: