Sci-fi's Unsatisfying Sequel Problem

Recently I finished reading the sequel to Arthur C. Clarke's groundbreaking Rendezvous with Rama.  This second book in the Rama series, Rama II, details a second excursion to meet a second mysterious alien ship.  The book itself is rather uninteresting.  It follows up on many of the themes of the original novel, but fails to captivate in the same way.  Rendezvous with Rama focuses on the unknown ship's construction as the cosmonauts attempt to divine its origins and purpose.  This all established, Rama II introduces a new set of cosmonauts and delves into their interactions instead.  We do get more descriptions of the interior of the Raman ship, but they are largely just developed detail of the first novel rather than any extrapolation into new material.  There are a few revelations, but these are never built upon.  For example, one of the cosmonauts (in an unresolved treachery plot) runs across doppelgängers of the first crew to visit the first ship.  This should be a major turning point of the novel because it reveals that the first interstellar ship is in contact with the second, despite there being no evidence of any sentient alien crew, and also that the "biots," or robotic alien animals that both human crews encounter may actually be replications of species that the ship had previously been in contact with.  The discovery goes largely uncommented upon.
The problem with Rama II is one that pervades a certain era of sci-fi.  Clarke's other noted series, 2001 and its sequels suffered the same fate as well as Asimov's Foundation series, among others.  These series all began as brilliant one-off novels that foundered and lost their way.  Rama II was unnecessary and I cannot imagine that the remaining sequels get any better.  In fairness, Rama II has the potential to be a good stand-alone.  The character development is good and there is the making of a good thriller plot within the novel.  However, as a sequel to Rendezvous with Rama, it breaks too sharply from the thematics that made the original striking.
I read the first novel long after I had begun reading hard sci-fi such as that written by Kim Stanley Robinson, but is easy to see the influence that Rendezvous had on Robinson's work.  That novel is deeply invested in the logistics of interstellar travel and the difficulties humans would face when trying to interact with a true alien culture/technology.  There is some character drama in the book, but it is largely in service to this main theme.  Rama II loses that focus.