Calibre conversion x to KF8 (MOBI) not readable on Kindle 4

[Grom]

Hi, I'm sure this topic has been discussed previously, I searched extensively before posting this but couldn't find a specific answer to my problem. I'll make this brief.

What I do:
Convert any type of book (zip, epub, txt) to Mobi selecting the "new" from the 3 options (old, both, new).

Result:

• Mobi(KF8) file is readable in calibre viewer
• Mobi(KF8) is not readable on my Kindle 4

What else:
Downloading a demo Mobi(KF8) from the internet, i copy it directly to the same device and it is readable.

I am running the latest firmware 4.1.1 on my Kindle
Yes I know AZW3 works

I'm just curious if this is a known Calibre issue, any book i convert via it's plugin to the new format doesn't work.

Thanks in advance for the info