I've been experiencing the same problem a few times now. My leader in /overseer_elect/leader stops processing the collection queue at /overseer/collection-queue-work. The queue will build up and it will trigger an alert in my monitoring tool.
I haven't been able to pinpoint the reason that the leader stops, but usually I kill the leader node to trigger a leader election. The new node will pick up the queue. And this is where the problems start.
When the new leader is processing the queue and picks up a reload for a shard without an active leader, the queue stops. It keeps repeating the message that there is no active leader for the shard. But a new leader is never elected:
ERROR - 2013-12-24 14:43:40.390; org.apache.solr.common.SolrException; Error while trying to recover. core=magento_349_shard1_replica1:org.apache.solr.common.SolrException: No registered leader was found, collection:magento_349 slice:shard1
ERROR - 2013-12-24 14:43:40.391; org.apache.solr.cloud.RecoveryStrategy; Recovery failed - trying again... (7) core=magento_349_shard1_replica1
INFO - 2013-12-24 14:43:40.391; org.apache.solr.cloud.RecoveryStrategy; Wait 256.0 seconds before trying to recover again (8)
Is the leader election in some way connected to the collection queue? If so, can this be a deadlock, because it won't elect until the reload is complete?