PDF to KFX on Kindle Scribe

I'm converting PDFs to print replica KFX files for my Kindle Scribe and I am having an odd issue with covers showing up on the Kindle for all files.

Here is my process:

1. Convert the PDF to greyscale using the following command:

2. Create kpf file using Kindle create by making a print replica file.

3. Use the following command to convert the kpf to kfx:
I thern import the file into calibre and sync it over to the Scribe.

Sometimes I get covers and sometimes I don't.

Any idea on how to force the covers across?