While insisting that his side's performances have not been particularly bad, Hummels is unable to explain why they are struggling to pick up points.
"We've not been worse than any of our opponents, yet we're still losing one game after the other," the Germany international told the club's official website.
"This is currently a really, really difficult phase in which in the Bundesliga everything that can go wrong is going wrong. It's slowly starting to get really difficult to find explanations."
Dortmund are back in action on Tuesday when they visit St Pauli in the second round of the DFB-Pokal.