It's really hard to give your characters bad luck. It's not half so hard as to give them good luck, particularly after the opening scenes and through the climax, because that looks like author favoritism, and we give more slack to bad stuff, which isn't.
Though it can have the reader thinking "you, the author, just did that to hose them down" and that's not good.
Bad luck is, by definition, a coincidence. Coincidences, particularly those late in a work, betray the hand of the author. And they dislodge the action as it rises to a crescendo in the climax. The unleashed forces need to duke it out without irrelevancies.
Then, coincidences are also realistic. They add local color and muddle and all those things that make a world rather than a stage setting. It's tricky to sidle them in without disrupting the plot, but then, one of their purposes can to be make the plot look more like something that could happen.
Helps if there are clues pointing to the possibility of its happening, but I'm still wrestling with it.