[mazadaz]

another thing that it could be is that the fit round the legs of the nappy is altered when lying on their side and if there is a far bit of bulk between the legs (as in a night nappy) then it is less flexible and more likely to gape a little when on their side. This happened a bit with my DD for awhile so I boosted the outside of the nappy and reduced the boosting inside and sometimes I double covered if I could be bothered (using wool covers)