When they fit, the double poppered (poppers at the waist and the leg) nappies fit beautifully
But when you're between poppers, or have a baby with skinny legs and a podgy tum (or vice versa) it can be hard to get a good fit.
A good, snug fitting PUL wrap can sometimes help here, holding the nappy in place in those places where it gapes - but obviously this defeats the point of having a pocket nappy
It may be that your LO just needs to fill out a bit more before the nappy will fit him. Is he fairly long & skinny? Nothing wrong with that at all, just his dimensions will be completely different to a 13lbs short, chubby baby like Erin was & who has always had a pretty good fit with WN nappies, both pockets & fitteds.