Bought a "2nd hand" Dracaena that was doing great for the first 5 months, but I've been struggling with it since the spring and I don't know how to make it better. I repotted when I got it, as it was in original nursery pot, and I got two new growths, which I was thrilled about. But then it started to have leaf die off, which has continued for months, and it's lost 2/3 of it's leaves. No leaf wilt, just browning/yellowing that starts on the tips and then progresses. Sometimes the sides look like scorch marks. Some leaves also look less opaque, and I can see the vascular system. No desiccation of the stalks or mushy stem. I have a smaller Dracaena next to it in the same lighting conditions and watering schedule, and it's doing fine. I water with plant food once or twice a month. I have tried more water, less water (I usually water once a week, or 1.5 a week), more sun, less sun (it's at a west-facing sliding door with sheer curtains) and nothing has helped. I've examined it with a flashlight and I don't see any sign of pests, no visible bugs or webbing, but recently I did see a couple of tiny, tiny dots at the base of one shed leaf that might have moved, or might have been my eyes playing tricks on me. There are some beads of sticky clear liquid, but I don't know if that's from pests or just guttation. I just sprayed it with insecticidal soap in case the tiny dots were mites. I'm at a loss for what to do, and I really don't want this plant to die. Any advice or diagnosis? Please help!! Thank you. Healthy 1 is when I first got it, Healthy 2 is when it first started to thin out, Sick 1 & 2 are the current status. The rest are individual leaf damage.