Here's what we see happen in our Jenkins instance every once in a while:
A build (let's call A) will get stuck on the "Archiving Artifacts" step. (The artifacts are no bigger than maybe 10MBs total).
Each other build of that same job will be blocked waiting for that build A to finish archiving its artifacts. If I cancel the 'stuck' build the other builds finish archiving their artifacts almost immediately.
We're not building anything, this instance is solely for running Squish tests. The artifacts are the results of those tests. All tests call a single paramaterized job for their build. This build is concurrently running on separate slaves.
These jobs are all running on Windows slaves. The master is a linux box.
I'm not able to find any logs that have Archiving Artifact output other than the main console logs. Let me know if there's something I can attach.