Kindle Fire won't read my converted .MOBI files

Using Calibre 0.8.41 on a Mac.

I converted some CBZ files to .MOBI. I put them on my Kindle 2G and the files opened fine. Then I transferred these same files via USB to my Kindle Fire and they won't open. I've tried using File Expert (not Explorer) and I can't open the files. It say 'There is no handler installed...'. Then it says there's no handler after trying to find one. And yes, I did restart the Kindle Fire with no luck.

In Calibre, I had taken the CBZ and did a default .MOBI conversion. I saw threads here and removed the [PDOC] tag with no luck. Then I also changed that tag to [EBOK] with no luck. But these suggestions seem to address when the files are viewable as DOCs not Books. In my case, the files don't work at all.

I went to the Calibre Preferences and removed the [PDOC] default w/ no luck. I also converted MOBI to MOBI w/ [PDOC] removed w/ no luck.

To make things easier, I took a very simple PDF file and used Calibre to convert to .MOBI w/ [PDOC] removed. Even this doesn't open. What am I doing wrong? I can accept something odd w/ my CBZ/CBR files but even a standard PDF file (of pure text) doesn't work.

Any help is greatly appreciated. I want to use .MOBI because it's a bit smaller for CBZ files and that'll give me more free space.

Thanks in advance.