They've had a few injuries to star players like Reus and Gundogan, amongst others. Aside from that, they've been really poor finishing chances in the league. Immobile has to shoulder quite a bit of the blame there, as he has missed an epic number of good chances in league play. He's been very wasteful this season. On the other hand, he's been quite good in the CL. It's only 10 games or so of his career there, thus far, so it's hard to say how he'll work out, but right now he's a massive downgrade from Lewandowski.