Hello there,
The lock already exists. Example: Ken's quest won't happen until after you have done Sam and Anna has been done raw at the studio. Sam's quest is not dependent on any other, except her own if you decline her.
Have you spoken with her when she is at her Livingroom, are you able to take pictures of her topless at the beach and at home of her ass. If you moved quick through dialog its well possible you declined her.