I am trying to hang curtain brackets in a very old apartment I have just rented. The walls apparently have been repaired many times over the years, as I can't get the screws to hold in the walls. I've tried drilling holes & using anchors; I've also tried screwing directly into the plaster, but the holes just crumble and the screws don't hold. I also think there is a hollow window box around the window, so the plaster (or drywall?) doesn't go very deep. Any advice?