It looks like “tache” can also mean a spot, like a a spot on television or in advertising. Could that definition fit better in your context? “I wasn’t able to make the spot work well?”
It’s french. I couldn’t work on the spots. Was it the rug cleaner guy? Or I couldn’t work on the tasks. . Really depends. “Je n’arrivais ( I couldn’t ) pas a bien fonctionner (work well) Not work well, les taches (spot or task). Fonctionner actually means function but it depends on the sentence structure.