Long story short, sometimes, a snapshot would hang, neither finishing nor failing, and canceling it also has no effects.
Often, its a couple of bad nodes for whatever reason.
This little tool pokes /_snapshot/_status and prints the list of nodes that
have snapshot shards stuck in INIT. Often you'll want to restart those bad
nodes.
Grab it from the latest release, or, on macs:
brew install caarlos0/tap/es-snap-hanging-nodes