“First, establish that your vision is up to par and corrected – preferably to 20-20. Ask your AME what your numbers are.
Assuming everything is OK there, try changing your scan technique. Many pilots sweep the horizon and have difficulty spotting traffic. Instead, look at one 30 degree sector of view at a time and concentrate your attention in this sector for about 5 seconds, then move on to another sector. This technique makes it easier to spot objects that are moving or objects that don’t belong in the picture.
Pay particular attention to the areas close to the horizon. Aircraft that you see in the distance that are close to the horizon are at an altitude close to yours.
Once you identify other aircraft, visually keep a close eye on them to identify their direction of flight. Remember that aircraft which show no relative movement vs. yours are likely on a converging course with you.”