Next book stalls

I don't know why I've only gotten three chapters + prologue into my next book. Perhaps there really isn't a story there. Perhaps my frustration with the agent search for book #1 is the obstacle. Perhaps I've only got one book in me although I don't believe that. I know the characters and have a vague idea of the plot, but it seems to stall each time I think I've finally captured that elusive thing that drives the story and its teller onward. I need to practice mega-author Nora Roberts' advice to herself and other procrastinating writers: ASS IN CHAIR!