SharePoint search indexing

[Jacques van der Hoven]

What are all the points to consider in terms of SharePoint Search Indexing? 

We're dealing with an issue where results from subsites are not appearing in the search results even though the content has been present for quite some time and the site is searchable. 

Security is one consideration, but that shouldn't be a problem in this case. 

If you're using a custom column based on a custom open Term Set, should you expect items tagged with these keywords to show up in search results? What would prevent this from working? 

[#2]

This article provides a comprehensive description of common points to consider:

https://support.office.com/en-us/article/Enable-content-on-a-site-to-be-searchable-2a420d01-4706-4d2...

Besides the information in this article, I've ran into a few cases where colums somehow got corrupted over time causing content based on these columns not being indexed / appearing in search results. This was mainly an issue in development environments where provisioning / updating the columns the wrong way led to corrupt field definitions.