I think everyone's comments are pretty good, so I just wanted to add that I struggle with the same problem. I frequently look at every Throughline and think, "Well, it's about their attitude towards the Problem, right?" In the end, I think every character is powered by belief--belief in their Purpose, belief in their Methods of Evaluation, their Motivation, or their Methodology, what have you. But in Situation & Activity stories, that belief is funneled through the problematic situation or activity. Because they are in a bad situation, their belief causes trouble. Whereas for Fixed Attitude, their belief causes them to have a troublesome mentality.
Perhaps as an experiment, you can try and figure out the difference between Pursuit under Situation, Pursuit under Activity, Pursuit under Fixed Attitude, and Pursuit under Ways of Thinking. If the OS Problem is Activity-Pursuit, and the IC Problem is Fixed Attitude-Pursuit, how do those manifest differently in the story? I'm not totally sure about the answer myself, so it's probably worth considering.