When I "go to bed" with Sylvie,the action box is empty.Is this normal or a bug or should I increase her lust
When she is in bed you can click on different parts of her body:
Breasts to fondle, lips to kiss, clit to tease or pussy to penetrate (with your fingers or dick based on which mode and position you are in.)
Simply click on those spots again and again to repeat the action.
(That is also how you train her sensitivity which maxes out at "Mega High.")
On the right side of the screen there is pink and blue bar, they represent hers and your lust level, once they fill up, you, her, or both - cum.
Some action may be locked behind the: "intimacy" and "lust" she has, you can see those in the "status" window outside bed time.