Custom font will not show on device

Hi all

I wonder if I could get some advice again. The problem is driving me crazy.

It is to do with custom fonts in my file not displaying on the Kindle Fire.

I have created an epub file (for later conversion to Kindle). Custom fonts work fine as an epub.

When I run the epub into the desktop Kindle Previewer, again the custom fonts display fine. However, when I load the mobi file that is churned out by the previewer, onto my Kindle Fire, it has reverted to default fonts.

Does anyone have any advice on what I should check for, or what might be the reason for this?

Thanks in advance
Dan