Once a player has been tackled, they must get up and play the ball.
Often when a player has been tackled close to the try line, they will often attempt to make another movement to ground the ball for the try.
However, if they have been tackled, the referee will not award the try because it is seen as a double movement.
The ball and the player have been grounded before the second movement for the try.
However, if the player is in the process of being tackled and the ball has not been grounded before the try line, then they can make a second movement for the score.