Does your LO do big wees suddenly? It could be that the absorbent part is natural fibre (think it's organic cotton), which can take a bit longer than microfibre to absorb liquid. If he does a big wee, it might flow over the cotton and reach the leg elastic. If there was enough wee it would leak. You could try adding a thin microfibre booster over the top as mircofibre absorbs a lot quicker, but don't put microfibre next to LO's skin as it can irritate. Maybe go with a very thin microfibre booster and then a liner over the top.
Or it could be because the leg elastic is different on Elementals/Organic AIOs. If you look, it's sewn in a different way to Flips or V3/V4s and that just might not suit your LO as well.