Could be one or two things, either they fit but are not a good fit - how near the top of the weight range is your little one? If chubby it could be outgrown before the upper weight limit reached and while it 'fits' isn't a good fit thus it leaks, maybe by the bulk of the nappy pulling it away at the legs.
Or are you adding boosters inside the nappy, and it's making the fat nappy pull the elstic away at the legs so the easiest wee escape route is down the leg & out onto the clothing = wet outside the wrap looks like wrap is leaking (cure stick the boosters between nappy & wrap rather than inside the nappy)
Or is it that being secondhand you have no idea of the history, they could have done 2 babies already and infact be a bit worn out? I'd not buy a secondhand wrap for a baby that is mobile purely for this reason, they get a lot more wear and tear.
I'm sure the fabric conditioner won't have anything to do with it.
Thanks mears, I have looked at that info before, but it is useful to have a re-read
It is probably too late for a couple of them, but I shall give the tumble drying a go and I will stop putting the wraps in with DH's shirts when I wash from now on as well (it is only him that likes the fabric conditioner anyway as it makes his shirts easier to iron )
Ariela - the wraps are definitely leaking through the fabric and not from the legs. I haven't changed my nappying habits recently anyway so no reason why it should suddenly start happening.
I had a couple of wraps start leaking through the fabric. Tumble drying seemed to solve the problem with one of them (a motherease airflow) but not my onelife. Also it didn't revive my fuzzibunz that started to do the same thing. Still haven't worked out why they went like it in the first place as I'd bought them new for my son, but then I discovered I'd been washing higher than the reccomended tempreture, so I put it down to that.