I'm a newbie. I don't know what is wrong with it. But, I'll pose some questions and give some comments, anyhow.
Could the plant be signaling you that it actually *needs* to be repotted? You said that you've had it over a year. It's usually stated that CPs need repotting after a year or so. Some sarrs, I understand, don't like constant water. Could the potting mix at the bottom of the pot be anaerobic? If no other plants are showing signs of distress, if the plant has not been subjected to any trauma, and no cultural practices have been changed the only difference between when it was doing good and now when it's not doing so good is.....time. What does time affect? ....the growing mix. Just a thought.