I have a 4 year old ficus triangularis. Recently, a lingering issue--which hasn't caused any issues in the past--has gotten worse. I have not moved it, and I maintain a very regular watering schedule, which I vary according to the season. Leaves are curling and turning yellow. There are brown spots on the backs of some leaves. I've tried cutting back on water, and also watering more frequently. I have a soil meter to measure moisture, and the soil is usually dry in the top 2 inches. I've also flushed the soil because I thought it might be too acidic. It's not adjacent to any plants, and it lives next to a permanently shut window. So I'm not sure how pests might access it (so I'm assuming that's not the issue--but I could be mistaken). I suspect something else is going on, but I have no idea how to troubleshoot this issue. Please advise.