I think it's a combination of factors.
* We seem a lot less hungry or motivated in many, many games. Something I personally had expected before the season began. Last season basically everyone wanted to prove a point, show the world what they were capable of, (almost) every single game. We were underdogs, we're not anymore, quite the opposite. Some/most players may get a bit complacent, we're underestimating the opposition, they "know" we're technically a better team compared to least season which leads to less of an effort being made, etc.
* Cl games mean players are saving themselves. Only human to want to be at your best for the "important" games.
* We're missing Chiellini, Asamoah, Pepe for long periods. And during this period, there have been fitness issues with Vucinic, Marchisio, Vidal, Pirlo,... That's a lot of quality you're missing right there.
* Some moments in time you're lucky, other moments you're unlucky. Right now, we're unlucky. In terms of referees, in hitting the post instead of scoring a goal, in getting punished for every single mistake we make (De Ceglie, Buffon, Vucinic,...). Every single team out there makes mistakes throughout every single game, very few teams get punished for basically all of them all the time. But we seem to be going through such a period right now.
I have to say that I don't think we were in better shape last year though. Let's not forget that we had some less than impressive periods last season as well, despite the unbeaten season: Only 3 wins out of 9 games in Feb-March.