The answer to that question can be used for a lot of shows.
Say you’ve been following a show along since the beginning - maybe The Americans, maybe Bones, maybe Enterprise, maybe Castle, maybe Burn Notice. The show may or may not have ever been amazing, but it has been pretty good. Good characters that you care about. Good plots. Some weak episodes, but a lot of good ones.
The one season, the quality drops. Stories seem half finished, plot points get dropped. Characters start acting out of character. Or an annoying character trait that was merely a small part of a person become their major characteristic. Or the show starts a new story arc that just seem stupid.
So what do you do? Often, you wait it out, see it the writers can get back on track. Maybe it’s just temporary. Maybe they’ll manage to salvage the season and write something that will make the weak episodes, if not good, at least not stupid.
But at some point, you finally realize the creators have lost it, and lost you. But the thing is, you can’t always tell while it is going on when that point is reached. Or even worse, you realize they have been writing a different show than you thought - where the ostensibly good guy is really a jerk, where the creators aren’t writing about a lovable scoundrel, but an actual sociopath, AND they expect you to root for him.
For example, Burn Notice has so many places that could be considered breaking points. Did the show lose it when Michael helped Simon escape? When Ansen came on? When Michael murdered his CIA boss? When he started working for James?
These annoyances just add up until one day, you realize you’re done. Of the five shows I mentioned, I reached that point with four of them and just quit watching. I can’t even watch early Bones episodes any more, but not so with Castle or Burn Notice.
The Americans aren’t there yet, but one more year like this (or worse) and I may retroactively hate this season as well.