Forgot to save the file

src/main/java/tsp/headdb/listener/JoinListener.java

@@ -14,6 +14,7 @@ public class JoinListener implements Listener {
     @EventHandler
     public void onJoin(PlayerJoinEvent e) {
         HeadDB.getPlayerdata().set(e.getPlayer().getUniqueId().toString() + ".username", e.getPlayer().getName());
+        HeadDB.getPlayerdata().save();
     }
 
 }