There can be many causes for asynchronous indexes to not get updated in AEM.
Some possible causes are:
First step in solving such an issue is to debug the root cause. Once you know the root cause, then you can decide whether reindexing is required or not.
On AEM versions prior to 6.2, all indexes used one lane "async". That index lane's status could be viewed in the IndexStats MBean:
In AEM6.2, there are two index lanes:
You can view the IndexStats MBeans to see the status of indexing and identify if indexing has failed or if it is taking a long time.
The key fields to refer are:
If you see an error listed in LatestError, then you have to work on fixing that error to make indexing work again. Contact AEM Customer Care for assistance if you are unsure of next steps to resolve the issue.