A sore point
Definition 1 of 1
Pro Tip 1/3Emotional Sensitivity
Use 'a sore point' with care, as it refers to subjects that might cause emotional distress or provoke an argument. 
I avoided talking about the divorce since I knew it was a sore point for him.
Pro Tip 2/3Conversation Caution
When discussing sensitive issues, 'a sore point' alerts listeners to tread carefully to avoid upsetting someone. 
Be cautious when you discuss her performance; it's a sore point.
Pro Tip 3/3Context Importance
Understanding the situation or history behind 'a sore point' helps to grasp why the topic is sensitive. 
His lack of promotion is a sore point because he's worked here the longest.