In the middle of the war, we see Melisandre seducing Stannis into having sex with her, then she gives birth to a "shadow demon" or popularly known as "Demon from the Semen". Then she sends this demon to kill Renly.
After Stannis gets defeated in the Battle of the Blackwater and goes back to Dragonstone, he asks Melisandre to produce another "Demon from the Semen" but she says he's too weak. She has found other ways of killing off enemies and searches for Gendry as he has the blood of the King. She brings him to Dragonstone, seduces him, ties him to the bed and puts leeches on his body to extract his blood. She then puts those leeches in fire saying names of people she wants killed and those people then die.
My question is, since Gendry is King's blood and he was clearly seduced by her, why didn't she have sex with him and produce another "Demon from the Semen"? After all, he is strong enough and has King's blood in him.
Also, what are the restriction, if any, in producing this Demon?