Do u have a boy?! Make sure the willy is not pointing up in the nappy. Seems basic but makes a big difference lol it needs to be pointing down.
But definitely size up also. I pretty much went NB - size 2 - size 4 - size 5. No real downside to a bigger nappy (as long as it obviously fits) just holds more so you don’t get the leaks. (Obviously still change as regularly as normal)
You can try sizing up like others have said - but also read somewhere that leaks up the back could be due to pulling the tabs too close to the middle when doing up the nappy.
Could be time to size up but also with my son we constantly changed brand as different ones worked best for different ages as he constantly changed shape as he grew!
Size up and see if that helps