Using Splunk Enterprise Server 7.1.2 downloaded from docker hub. I added a folder via: Settings/Add data/Monitor/Files and Directories added a folder that had several files in it. I had assigned it to a new index, and I can see the index has a size of 5.65G, so I'm confident that they have been indexed, but searched for words that I know should return a lot of hits, but no results are returned. I even ensured that I selected "All time".
I'm glad you figured out an answer to your query! Would you mind posting an answer below detailing how you solved your problem and then approving it? That way, other users would be able to learn from your solution.
Thanks for posting!