The last time Dortmund won the Bundesliga was in the 2011-2012 season and after that Dortmund no longer won titles in the Bundesliga including last season where Dortmund were only able to secure runner-up positions in the Bundesliga. Speaking of players, I see that Dortmund always have the best players in their squad who they will eventually sell to other teams for more money.
Because Dortmund is very good at training players and making players very talented as in several examples that have been seen by everyone, so Dortmund's defeat last season in the Bundesliga was not on the factor of their players, but on the factor of Dortmund's luck which was still very thin.
The Bundesliga league is difficult; Bayern Munich maintains complete dominance of the league with no competition. Borrusia Dortmund is an exceptional club, just like Bayern Munich, yet they have consistently failed to win trophies like the Bavarians, and settling for less has become a habit for them. The previous season, though, Borrusia Dortmund was extremely near to winning the league title, but scrambled in the final game against Mainz, all Borrusia Dortmund needed was a win, but they failed, and Bayern Munich lifted it as usual without any obstacles on their way.