The problem not only are the away game results but especially the weak showing against relatively weak teams. Sure they won the last 3 games against bottom of the table teams, but in the first round already didn't look convincing against the same teams. Against Heidenheim they just played 2:2 at home, and now an even worse 0:0. If you can't beat a team like this it won't be enough to make it into the UCL, plain and simple.
These are crucial weeks for Dortmund, against these teams they MUST score because at a later stage of the season it only gets harder when they face teams like Bayern, Leverkusen, Leipzig, Stuttgart and so on.
Heidenheim is not a weak team, they are strong especially when playing at home and the draw against Dortmund is quite natural because on the other hand Dortmund's strength this season is not that strong and in my opinion it is not even stronger than last season.
If compared to Bayern Munich, Leverkusen, Leipzig, Stuttgart, the Dortmund team is not stronger, so with the fierce competition this season, I quite doubt that Dortmund can finish in the top 4 of the final standings.