That is because a scout's knowledge isn't infinite. As you make the scout search for newer countries (which aren't already in his knowledge), the countries which are already scouted slowly reduce.
That is why there is generally a two step process to scouting. First, use a scout with a very high adaptability rating to scout a new country (his CA/PA ability isn't that important). Once the knowledge of the country is present (should be visible in your club's scouting knowledge), then use a scout with good CA/PA prediction to scout the country for good players to get. Even then, knowledge reduces after a while, but by then, you'd likely have gotten your players.
If you want make your scouting knowledge very high (not necessary at all, btw), then you'll need higher number of scouts in order to effectively make use of it.