What am i doing wrong??

May be a little tmi. So my husband and i have two beautiful children together. We have a fairly decent sex life but since we’ve had our oldest he stopped going down on me as much, no big deal so i don’t get it as often? Okay. But since having our son recently during my pregnancy at the end and after having him he has not wanted to go down on me... I’ve begged and pleaded for him to because i miss it but he won’t. I mean any time he asks for me to go down on him i do.. but lately i have no interest because he refuses to return the favor.. he uses the excuse of he’s to tired. And great, i get that! But i am tired to, with the kids and work but when you ask me i do it with a smile on my face because i want him to be happy. And the few times i haven’t wanted to he tells me come on take care of me... please help.. is there something wrong with me? Am i not doing enough...? Is he no longer attracted to me that way anymore??? I want nothing more than for him to go down there...