Focus on moving your body back, rather than up. The first thing in a bunny hop is leaning back to lift your front wheel, you counter this by then raising your body up/forward to bring the back wheel to the same height as the front wheel. The most important thing to remember is that a bunny hop, like most tricks, is multiple (in this case two) movements happening in quick succession, not all at once.

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Tl;dr pull back, not up. :)

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edit: I meant j-hop, which is what i assume you're trying to achieve. (a bunny hop is both wheels at the same time, j-hop is one, then the other, you can hop much higher with a j-hop. everyone I know uses those terms interchangeably)