Football is the ultimate team game IMHO because it takes a team, from ownership all the way down, to win and win consistently. So, I totally agree that it's not just "one thing." For example, losing (and losing in epic fashion) is not just simply the result of 1 or 2 guys on offense and/or on defense going on the IR list (even if they are good players).
Back to the passing game, briefly. Fitzpatrick is still a limited NFL QB. Chan designed a system to suit his skills, but that system has been decoded now. Fitzpatrick's early success has come apart and he's struggling in the same offense as he operated early in the season. Pre-snap reads, spreading the defense horizontally, and getting the ball out of the QB's hand in under 2 seconds isn't creating the same space as it did early in the season and the production has unquestionably plummeted.
But, yeah, a few key injuries, a garbage defense, etc, don't help the situation any, of course.