I found this article https://www.fileseek.ca/Discussions/View/text-search-on-docx-and-pptx/?ID=35fc91de-1293-4b23-822b-e97e0cb2203e#last which implies this issue was resolved Jan 2015. However, I have plenty of docx files and none are successfully searched.

The "Preview Handlers" includes "docx" with the location of my WINWORD.EXE.
The cited article refers to "offfiltx.dll" and I find it present under C:\Program Files\Common Files\microsoft shared\Filters.

I'm running Fileseek Pro 6.1 and using Microsoft Office Professional Plus 2013.

Operator error. I found that the "Process file contents using File Handlers" had been unchecked. Checked that option and it works as expected.