The most likely cause for this is your created legend is at the same position as your franchise player.

The legend will automatically be chosen over the franchise player. Also, currently the game will not allow both to play in the outfield.

You can set the status of your created legend in the "My Legend" section of your team. You can choose to have them be your designated hitter, or you can place them on inactive.

Make sure the player currently in that position is not locked. Locked players will not be replaced.

Also be sure to check your Gameday strategy's roster decision setting. Depending on your current setting you might want to change to current or potential to ensure the franchise player will play.

Finally, if your franchise player isn't better than the player you have at the position it will not be played. You can always swap in a lesser player to the position (which will let the franchise player be better and play their games instead) and then swap back after your franchise player games have been used up.