Sex issue

Sex is a huge issue in my relationship right now... My boyfriend and i have sex once a week and i want it more. I've brought it up to him before and he said "i just don't feel like doing it". We used to have sex everytime we saw each other now I'm lucky if i get it once a week. And his ex girlfriend (my friend and his daughter's mom) says its really unusual for him. What am i doing wrong or why doesn't he wanted to have sex with me!?