I have a pothos plant in my kitchen. It's about a year old, and has been healthy up till the past month or so. Now the leaves in the center of the plant are yellowing and dying off (no splotches or anything like that). The surrounding leaves and the leaves growing on the tendrils of the plant appear to be fine. I've watered it once a week since I've had it, there's no sign of bugs or anything else wrong. It's in a sunny window, facing mostly north. Any ideas??