French Braids

ok, i hope that i can explain this properly.... I have very straight hair that falls between my shoulders and bra strap, so it is long enough to french braid. i braid my hair when it is damp, and it goes smoothly enough BUT i find that after my hair dries the braid gets puffy and loose at the nape of my neck, this gets worse over the course of the day as i move about. it can look quite odd (imagine looking head on - there is a fairly even silhouette from the crown down, but at the nape, there is a squarish protrusion on both sides). I have tried to tighten my tension, but this doesn't seem to do anything but put off the inevitable. also i have tried putting in an elastic and wearing the ends in a pony tail instead of continuing the braid, i still have the loosening at the nape, where the elastic is. would appreciate any advice you can give. i am hoping that there is something i can do, and that this is not just a consequence of having straight hair.thanksj