Best Example: Sons of Anarchy
Remember how excited we all were after SoA's season two finale, which implied that the gun-toting biker club would take the action to Ireland, for a face-off with the IRA? Well, they went there, but in a cruel twist of irony, the Ireland-set episodes instantly became the weakest arc of the series. Instead of the soul-searching and baby-hunting trip we imagined, most of the arc (save the excellent episode "Bainne") was dedicated to a schism within the IRA that, while bloody and violent, was far from engaging, mostly because we knew nothing about these new characters and conflicts and were given zero incentives to give one single fuck about them.
The fourth season is an improvement, but upon rewatch the club's involvement with a Mexican cartel, and subsequent, unwitting participation in a war with a rival operation, was just as distracting as the IRA. It wasn't until the currently airing fifth season that the show fully found the footing that was so firm during its first two years, by showrunner Kurt Sutter simply refocusing all of the narrative stakes back on the characters we'd come to know and love.
Sure, there are new characters like Jimmy Smits' pimp with a heart of gold or Harold Perrineau as ruthless boardroom gangster Damon Pope, but every conflict or threat posed is only significant in the ways that it affects the increasing erosion of Jax's (Charlie Hunnam) soul and his relationships with his family and his biker brothers (R.I.P. Opie). The drama has returned to the Reaper table, where it rightly belongs.