The conflict has to progress. To get somewhere and do something. So the conflict has to keep on building and getting places. Things have to keep on getting darker -- even if the hero is going from success to success, it has to seem to make things worse, so that the reader doesn't yawn and declare that of course he'll win this, too, since he's always winning everything. Stakes keep rising. . .
At the very least, the plot has to take a sudden veer and go off into new and different direction. To keep things from getting too boring. If the conflict between the hero and heroine stems from a lie, even if the lie escalates -- if, for instance, the heroine thought it was trivial and realizes more and more how important it was to the hero, and how every time she doesn't confess builds up the hurt he will feel when he knows what she kept from him -- it can get a little monotonous. Having it explode into discovery and fury would shift the plot to new and different ground.
The quest can have a lot of issues with it, since the main character (with any companions) goes from place to place. Keeping on finding stuff and locations is not an escalating plot. More clues as he draws nearer the aim, and the more danger, too, as his object is better guarded, and the fewer resources as he uses them up -- but it's got to escalate.