My first thought when this happens is the pressing. A block can be stretched or creased at the seams.
Second is if the fabric is washed before it is used. If not, steam or spray starch can affect the fabric.
I will go as far as setting seams with a very light bead of Elmers Washable School Glue if I think there will be problems with certain fabrics. This works great for strip sets, HSTs, joining blocks into rows and rows into finished tops.