I believe you have to use the draw button on the game. As far as I know it then gives you a claim draw option if it is valid.
Draw by threefold repetition
Yes, one of the players must "claim" the draw by clicking the draw button when the game will automatically be declared a draw if it is a 3-time repetition. If neither player hits the draw button then the repetition can go on and the game will continue.
I just played a game on live where I had a perpetual check forcing threefold repetition. The server nerver seemed to pick up on it until we had repeated the position about 9 times. Are you aware of this?