AIOs ask a lot of the wearer, if that makes sense. A lot of things have to go 'right' to wear an AIO without leaking.
The most important consideration is also the hardest to manage: Correct sizing. For example, I'm right in the middle between mediums and larges for most AIO products. I can wear the mediums without leaks, generally, but they're less comfortable than the larges, which leak quite often.
With the diaper on, the second key consideration is fit around the legs. This is where most AIOs are prone to leak. If there's any of the interior lining outside the leg cuff, you'll have a leak. If the leg elastics don't form a good seal, avoiding leaks is almost impossible.
Finally, some fabrics are more prone to wicking than others. For example, some diapers tend to wick to the outside if wetness exceeds what the lining can absorb. So changing frequently is good practice. That's not always possible, and I've found that sitting too long in a wet AIO will almost inevitably result in a puddle when I'm ready to change.
I hope this helps!