Never able to finish him off?

My boyfriend has never gotten off from just a blowjob. I used to be terrible at them, but i have really improved, to the point where he asks for them most days! But no matter what I do, it always turns into sex. I even asked him to let me try to get him off but he couldn't help but ask for sex instead. Am i the problem? Is this even a problem to begin with? What do you ladies think?

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