I have cement and brick walls in my apartment. I am having problems hanging some new shelves I recently purchased from Pottery Barn. I have drilled into the cement with a 5/16 drill bit and placed plastic 5/16 anchors in before putting in the screws. The problem is that after mounting the shelves and placing items on the shelves, the anchors seem to be slipping out of the holes. Do I need larger screws, larger anchors, or perhaps a different type of anchor (metal or wood)? Help!!