Can you provide detail about how you are testing?
Perhaps you are running into the issue described here:
In short, if you are using "Send to Kindle", it will corrupt the ebook archive (by stripping fonts and messing up the encoding). This is not a supported way of passing the archive to a device.
If you are using one of the supported methods for transferring the file, then we'll need to dig deeper.
Cheers,
-Dan