Individual slots lock daily at the game time scheduled for the player in that slot. This occurs even if the player is not playing, so make sure you move injured players prior to their team's scheduled game start.
Individual roster slots lock when the team of the player in that slot is scheduled to begin their first game of the day.
Remember, roster locks involve add/drops while lineup locks only involve active/bench. For instance, when the first game of the day starts, your roster is then locked. Any players added after will display on the next day for daily leagues and on Monday for weekly leagues.