There are several reasons why a document can no longer be found. Perhaps the text you are searching for resides in a field which is not indexed by IDOL. This is often the case when people search for a word located in the title of the document as an example. What they fail to note that the title field is not an index field by default. Same goes with File Types searching. Here is an article I wrote on how to make fields index fields.
https://storage.googleapis.com/idol-doco/How do I search by file types in IDOL.pdf

The best to debug it would be to look at the IDX of the document indexed by CFS. Look for the word you are searching for and ensure that the field is an index field.

The Fieldtext parameter will allow you to filter search results by metadata. see documentation.
https://www.microfocus.com/documentation/idol/IDOL_12_2/Content_12.2_Documentation/Help/index.html#Actions/Query/ActionParameters/FieldText.htm