I wouldn't call it unfathomable complexity. It's really quite simple. Check the quest log. If the scene you're looking for doesn't appear anywhere in the log, then it means that there is some prerequisite scene which hasn't been completed yet. If the scene you're looking for does appear in the log, then it will give you the next step to take in order to view the scene. After that step, there may be more steps to take, but keep following the quest log, and you'll get there.
And remember, sometimes you may have to send Lisa home and have her sleep for a night or two without taking other actions. Sometimes scenes which trigger automatically won't trigger after you've just completed another scene, and having Lisa "sleep it off" clears the queue, so to speak.
Edit - Also, remember to check the outfit requirements for the scene. Does she need to be wearing the summer blouse and jeans? The black athletic outfit? Something else?