AZW3 Conversion hangs

Greetings,

I am trying to reconvert an AZW3 file to another AZW3 file to update the metadata before I put it onto my Kindle. The problem I am having is the conversion process seems to hang at the "Flattening CSS and remapping font sizes step" This hang happens no matter what format I try to convert to. I have tried both 64bit and 32bit versions of the software, changing the file type to AWZ and Mobi with the same result. I am using version 1.18 for Windows. Any help with this project is greatly appreciated.

Thanks!