To me it isn't a simple situation where you can single out certain players or coaches ("Brady is declining!" "McDaniels can't make adjustments!") as though if they got it together the team would start steamrolling everyone. What I see is a team playing slowly, tentatively, and without confidence, except for the defense against the Vikings.
I think it's due to a combination of poor game planning and adjustments (have they had a strong second half in any game?), inadequate leadership among the players (this is where they miss Mankins most, and similarly Spikes), trying to force in guys that aren't ready (Devey, Fleming, and Cannon at guard in particular), and injury recoveries (Vollmer perhaps most of all, Gronk of course, Dennard, and Dobson aren't where they need to be).
This is rough right now and I think it will be rough still this week, and Buffalo and the Jets will give the Pats all they can handle due to their intense focus on New England. However I actually view this situation with pretty good optimism for November and December. With the exception of DeGuglielmo the coaches and key players have enough of a track record of improving through each year to make me feel OK still, barring major injury.
Their margin is very thin, and it won't shock me if they don't win the division or even miss the tournament. If that happens it will suck for sure but if it does and yet I see Stork take over the center position, Vollmer, Gronk, and Dobson return to form (I'm not worried about Dennard), Solder and Collins get their acts together, and Jones and Hightower continue to be playmakers, I won't be crushed. I'll focus on hoping Jacked and Pumped crashes and burns, that we get a Harbaugh-free Super Bowl, and to watch Peyton walk off the field with another team's confetti flying on the first Sunday in February.