It could also be a magnesium deficiency - get some Epsom Salts. Btw, are you watering your orchids with well water by any chance? The well water here is high pH and also high in Phosphorus which can bind some of the minerals that the orchids need - like magnesium. You can test your water with an aquarium test kit from Petsmart. It's good to know the pH of your water, both tap and well water.
I water once a week with rain water c/w a tablespoon of Epsom Salts per gallon of water. Lately also been fertilizing with a fert low in Phosphorus (middle number).
