Are this players with umlauts (don't know if this is English too)? Like é è ê á ã ? You need to replace the é with an normal e and than it works.
Another problem is when the player is in a region where you didn't scout yet. You simply don't know the player. You can find him by searching manualy to the club and clicking him in the club screen.
Happens to me, as Genie loads a save game I don't think its about what countries are loaded. It puzzles me as well, I've never checked to see if those players appear later on though (perhaps the game creates them before they become visible?)