Might be the now infamous KDP
font stripping issues. They can be downright miserable to work around, but it can be done.
The link above gives a few rules that work for most folks. If they don't work for you, then my recommendation is to start by ripping out about half of your stylesheet at a time (but not the font parts) and seeing if a test book shows up with fonts in the online previewer. If the fonts are still gone, put that half back, and delete the other half (but not the font parts). Continue to remove half of half, half of half of half, and so on, until you narrow it down to a handful of rules that cause the problem, and take whatever actions are needed to avoid those problem bits.