why cant i get my sim pregnant with a fairy and a vampire!!???

so i try in the day and not in the night!?? xD the fairies dont transform right?? i mean, she has the wings aaall day xD and the vampire is one i met so i cant control him....i have to keep trying xD not going to give up ^^ and this is would be my second child, the first one tha dad is a fairy too