How to prevent reserve players go back to Team B.

Hi guys. Im managing barcelona and i called up 3 players from Team B. And when i hit the buttom forward to go to next match day those players disapears from my main team and goes back to team B. How can i stop that from doing so.

Move them first to U19 or U20 (depending what you have), hit continue and after that you can move them to first team.