By their very nature AIOs will not be as leak-proof as a seperate diaper and cover. There's only one layer of elastic to keep clothing from creeping inside the diaper and only this same layer to keep pee inside. That said, even an AIO (especially an ME AIO) should not leak frequently.
Did you prewash it enough? If you didn't prewash it much it may still be in those early breaking-in stages where all the cotton oils haven't been quite washed out so it's not as absorbant as it could be.
Make sure your ds's equipment is pointed downward, not up or to the side. Helpful in any diaper but a little more important in an AIO.
Make sure there's no tight clothing around the diaper that's likely to creep inside (such as a onesie for example).
Make sure to change frequently enough. I know for me, sometimes when I was out and about I would get distracted with other things and not change as often as I normally would so my little ones diapers would get much wetter than usual.
That's about all I can think of, maybe someone with more AIO experience can make more suggestions.