I've just solved this same problem. Basically the update() method of my service was not working through the efRepository.Update()
efRepository.Update calls _context.SaveChanges();
which should save all the changes to an object that originates from _context.getbyId();
My problem was, when I converted from my Model to the Entity (using automapper), it was replacing the entity object, and losing the connection to _context.
But when I just changed to just updating the properties of the entity, it works like a charm.... 5 hours of pulling out my hair...