Write scenes where your characters are frustrated today. They are irritated, but don’t know why. They are anxious, but can’t put their finger on the exact cause. These are the pacing-around-the-room-with-worry scenes. The scratching-their-head-trying-to-fit-the-puzzle-together scenes. Don’t let them solve the problem too quickly. Let them stew awhile. Later, a detail from the past could be the key that fixes everything.