We use 2mm Yamamoto neoprene with smooth skin on the outside to make our custom fit neck seals.  Neck seal must be tucked in so that the smooth skin lining sits against your neck, ensure that there are no stray hairs or creases before you have your zip closed.  The pressure In the suit creates the neck to bellow and this helps seal the water out.  Untuck the smooth skin surface fully before removing your neck seal so it doesn’t grip to your skin and hair.