Why do ISFJs have difficulties with deep conversations?

INTJ here. I have an ISFJ mother. We have conflict at times. I try to have deeper/analytical discussions of the conflict at times, and she always avoids them. Most of the time, she says something like, "I don't know, it feels right." Then I counter with something like, "Well, why does it feel right?" Then I blurt out facts which show that she may not be right etc and then I ask her to counter. She never counters and she simply leaves the conversation ("I don't want to talk about this anymore!").

Why? If her ideas/opinions are based on sound facts, she should be able to defend them. Please offer your insights. Thank you.

note: most of the discussions centre around right/wrong things to do/actions to take.