My 7.5 m.o. DS wears Bambinex Teddies in the day but has had a lot of leaks recently - at least once a day. He weighs around 22lbs ish (99th centile baby) and has been in the large size for probably a couple of months. I use the boosters that come with the nappies, and put them at the front of the nappy. We use fleece liners and Airflow wraps (large size - again, he's been in these for a good couple of months).
The leaks seem to be coming out of the side of the nappy, not the top. I wondered whether it was because his clothes are tight (getting dungarees to fit round the nappy is a bit of a challenge) but then we had a leak with his very generously cut Frugi dungarees. 
I change his nappy every couple of hours. Today it leaked during a nap and less than an hour after the clean nappy went on his clothes & sleeping bag were soaked. The nappy was pretty wet too.
We've not had any problems with leaking at night, when he wears Motherease one-size nappies and Airflow wraps. I'm less keen on ME though, and mine tend to whiff a bit (they were second hand) so I'd rather not use them during the day.
We had a similar problem a few weeks ago when I started using new liners. I put him back into the small size, but they were too small. Then I realised I'd not pre-washed the fleece liners and doing that sorted the problem out.
Any suggestions?