We encounter this issue several times, so I think it’s best to make a note here.
1. Check the content body source code
if below tag exists, that means the search visibility setting is not correct
<META NAME=”ROBOTS” CONTENT=”NOHTMLINDEX”/>
Always index must be checked.
2. Check Metadata Properties on Central Admin
Make sure the contentBody in Crawled Property is configured correctly.
Include values for this property in the search index must be checked.