You're welcome
Basically, CSS has a type of priority system (called a cascade). When you embed a font it sets the "base" font used by the book. But if the book specifies its own fonts with higher priority, then that will not work, in this case you have to tell calibre to remove the existing font defintions, to allow the new base definition to become highest priority. calibre does not do this by default, because it can cause some loss, think of books that use a different font for headings or handwriting or similar. If you dont care about that, then yes you can always tell calibre to delete font styling information.