What do your nests look like? Photos would be fantastic.
People use hay, straw, wood chips, wood flakes, Spanish moss, leaves, rags, carpet, sand, and who knows what else. I don't see anything wrong with pine needles or shredded pine bark. I suspect the real problem is that your lip around the nest is so low they can easily scratch it out when preparing the nest to lay. That's why a photo might help.