Perhaps share more pictures from the whole plant and the areas affected. How often are you feeding it? Do you have a TDS meter to keep track of how many ppm is put in and runs off? It's not completely necessary, but it's nice to have. A very low run-off ppm after watering might be a sign that you should increase how often or how much you feed, for example. Alternatively, a very high run-off ppm might indicate you're close to or already over-feeding it, which could burn your plant and lock it out of nutrients in the substrate/soil.
There's always the possibility of a pH problem (possibly too low), which could hinder the plant's capability of absorbing some nutrients. Those cheap pH strips are helpful, but a pH meter is recommended.