Formatting lost during epub to mobi conversion

Hello,

I have some experience with eBook publishing, so I am not a complete beginner.

I am new to using Sigil.

After spending quite a bit of time scouring blogs, forums, and FAQ's, I have yet to find an answer to my conversion issues.

When I use the Kindle Previewer to convert my epub, I get seemingly random formatting issues.

This is a problem through the Kindle Previewer and Kindle App on the iPhone/iPad.

Paragraphs are randomly indented.
Some-----text-----is-----fully-----justified-----where-----it-----shouldn't.

I am lost. Any sort of guidance would be helpful. Thank you in advance.