Leaking Happy Heinys

I've been using MCNs on my DS since he was about a month old (he's now almost 15 months). We've had our ups and downs, but mostly I've been very happy with all the nappies I've bought, most of which are OSFM.

Anyway, now that he's getting bigger, my OSFM Happy Heinys don't seem to be doing their job anymore. I can't get a good fit around the leg (although this was never a problem when he was smaller) and they always leak out the outside of the leg at the back. Any suggestions? Do you think it could be because I've extended it to the biggest size too soon (he's 10.5 kg).

Thanks.

It could be? Or maybe the elastic is starting to give a little so the seal around the leg isn't very tight? Or the PUL may need a hit in the dryer to fix any leaking problem?