I second that, looks like a possible nutrient deficiency. In garden plants, that usually isn't caused by a lack of nutrient, rather the wrong pH for the plant to absorb the nutrient. Do you check your pH after adding fertilizer?
If you are using a fertilizer that is designed for garden plants, you may not have all the micro (and macro) nutrients that you need (soil is usually well supplied with things that we don't have in orchid mix). Often it is calcium and magnesium that are limiting, so supplementing with those is a good first step. But lack of boron, manganese, iron, and a dozen or so other elements can also present problems. So, if you aren't already, you want to use a hydroponic fertilizer which has all trace elements in it already.