When they leak, how full is the nappy itself? If the outside is wet but the inside is dry (or at least should be able to absorb more), it could be repelling or it could be poor fit. If the nappies are repelling, they won't be absorbing liquid properly which means the wee would be slooshing around in the nappy. Even the best fitting nappy can leak when that happens! To see if it is repelling, lay the clean dry nappy flat or over a bowl and let some water puddle in it. Push down gently on it to simulate LO's bum in the nappy and the liquid should pass through into the nappy and away from the surface. If that doesn't happen, I'd use a full dose of detergent and lots of rinses to make sure anything trapped in the nappy is properly washed out.
If the liquid does go through okay, it could be a fit issue - sometimes one child can be a very different shape to another - I couldn't use all the same nappies on my two even though they had a similar build. Or babies have phases where they suddenly change shape and you need to start from scratch to get the fit right. Obviously check the legs and waist - give LO a good kick while watching for gaps. Also try and fit the nappy closely to the bum - if it's a poofy bum, wee can sloosh around instead of being pushed through into the nappy.
If the nappy does seem full when it has leaked, your LO might just wee more than you expect. How often are you changing the nappies? When big leaks happen it can seem like the liquid has some straight through the PUL when in fact it's come out of the legs and wicked up the outside, either up the outside of the nappy itself or up the clothes, which wets the nappy and then you don't know if the nappy was wet first or the nappy! You could try bigger/looser clothes too - if they're squishing the nappy, it can make leaks happen.
Unless you've been doing something really harsh to your nappies, I doubt very much that you'd have all of your nappies with PUL issues so I don't think it sounds like that. Tumbling isn't really recommended but if you use low heat occasionally it's unlikely to do much.