Cloth bums and car seats

My DD2 is 8 weeks and had just made the transtion into cloth nappies. (iwas given alot of disposables for her when she was born so i used them all up first) Now that she is in cloth (terry squares) i find she wont sit in the car seat properly. as in, her bum isnt against the curve of the seat and shes kind of slumped over. I dosnt seem to bother her but i know that car seat wont protect her the way its meant to if we have an accident.

What i dont understand is that DD1 was in cloth as well, in the exact same car seat and i never had a problem with the way she sat in the car seat.

Ive tried folding the nappies different ways but it doesnt make any difference.

Im looking for ideas on how to mayb fix it.

Hmmm I dunno, I use MCN not flats - but I've never had a problem with them fitting into the seat