When shards are lingering in an "unassigned" state in Elasticsearch, it can indicate various issues with the cluster's configuration or data. To identify the cause, Elasticsearch provides tools such as the cat shards API and the cluster allocation explain API. The primary shard may be unassigned due to delayed shard allocation, not enough nodes, re-enabled shard allocation, missing shard data, low disk watermark, or multiple Elasticsearch versions. In some cases, forcing allocation of a primary shard can solve the issue, but this should be done with caution as it may result in losing data on the original shard. To resolve unassigned shards, it's essential to address these potential causes and consider monitoring for such issues using tools like Datadog.