Hm. Sort of odd response from your guy. I have two theories:
Theory One: You are a very large gal, he's a very tiny fella and he feels like he truly can't mount you effectively. Or maybe he's large, you're a Mini Minnie and he's scared he'll crush you like Fat Joe sitting on a blueberry. Either way the geometry or physics are not clicking for him. This of course needs to be addressed as it's only a psychological issue -- any two organisms can hump. Believe me, you don't want to know what I've done with musk rats.
Theory Two: he is trying to be respectful of your boundaries and if you two are making out he probably has an erection and is trying to (for the time being) keep that away from you. Maybe he thinks you'l bug out. I would ask him about all this: Why sometimes? What makes it work sometimes and not others, honey? This will open up communication for you guys.
This whole issue smells like inexperience on your man's part and that's fine. It takes practice to make a make-out work. Chat it up, and together you will discover how to get your stack on.
Awesome. My new catch phrase:
Get yo stack on.
Nicely done, sir.
I soooooo want more details about this situation...impossible to get on top of her sometimes...what? I'm still laughing...