Just seen this on the Join Snap article here:
Known Issue : When the upstream Snaps of the Join Snap contains Copy, Router, Aggregate, or similar Snaps, it is likely that the data flow of a branch in a Pipeline gets blocked until another branch completes streaming the document. The Join Snap might hang if its upstream Snaps in a Pipeline has a blocked branch.
Workaround : Set Sorted streams to Unsorted in the Join Snap to effectively buffer all documents in all input views internally—this unblocks the document flow of all the upstream branches. The internal sorters sort the input documents from the input views into the local temporary stage.
I’ll do some investigating