If he's younger, could be side effects from medication, tiredness, stress, alcohol, could be distracted by something on his mind. Or it could be low testosterone which I think is the main cause with older guys.
It almost certainly has nothing to do with his level of attraction for you.
You're his girlfriend and you guys had sex a week ago and it didn't happen.
So far it's just a one off so I wouldn't worry about it.
Be sure your love making is just that, not a marathon or challenge to one-up (or out do) the “mechanics” in an attempt to please each other.
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