Elasticsearch Replica Shards Unassigned?


We recently set up Elasticsearch, and art the moment are running two nodes.

All seemed to be working fine initially, but I noticed today that some of the shards are unassigned:-

logstash-2016.01.15 4 r UNASSIGNED
logstash-2016.01.15 2 r UNASSIGNED
logstash-2016.01.15 0 r UNASSIGNED
topbeat-2016.01.15 4 r UNASSIGNED
topbeat-2016.01.15 2 r UNASSIGNED

I've had a look in the logs on the Elasticsearch servers and cannot see any errors that would give an indication as to why this is happening?

I'd appreciate any advice on how to start diagnosing what the issue might be.