I've only just started reading but I have to say there are only a few reasons why a coup can succeed.
1. The royalty is so corrupt and tyranical that their own soldiers rebel in addition to the nobility's as there are no reason for royalty to have less military strength than the nobles.
2. The royalty is so incompetent that they cant see that their subjects are plotting something and that even their own soldiers are not loyal.
3. They have Weaker military than nobility but that is highly unlikely as the royalty would not be royalty if their military was weak.
4. PLOT. HOLES.