You were probably just trying to buy more than was available at exactly that price right then. That's the kind of situation where an AON order can actually get in the way. I'd either try a smaller order or lose the AON restriction.
Sorry for the late reply. To be honest, I don't know. I don't think breaking it up would've made a difference, especially if your order is not set to buy entire order or nothing. And of course if you're buying on the ask, it shouldn't make any difference at all.
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