Actually

IB's use PUL and PUL has tiny little holes inbetween the weave/threading of the fabric so if you have a heavy wetter its going to come through quicker. It is not 100% waterproof

Have you strip washed them lately? how many boosters do you have in them?

Also the older your child gets the less they wee continuously...eg the closer to TT they get the more they hold onto their wee and do a big outlet of it which, if its enough will be to much for the nappy to absorbe and hence cause flooding/leakage