The lack of a quality striker is costing them. They wouldn't be in this situation now, if they had Lewandowski to finish off the many chances, they got earlier in the season.
Lewa has missed like 10 sitters this season already. Even Dortmund fans know he's not a great finisher. Maybe a dip in the transfer market is needed though.
Who is decent that they could possibly get? There are probably some decent ones in the Bundesliga. I think Volland only has a couple years left on his contract.
Mostly what they need to work on is team cohesion and confidence/motivation. There isn't any. They all look like they just met each other for the first time each game. Even with a decent striker, there's no guarantee you can keep coming back from conceding stupid goals in the first 15-20 minutes each game.