Are you proper squeezing the buffer off the swab too? At first I was only pressing it against the side of the vial, but it's much more effective to squeeze both sides of the vial and squish it on the way out.
You opening them before doing your swabbing? I had a bunch that were stingy on the liquid but waiting until the last second to open them after doing swabbing seemed to save enough from evaporating.
Are you proper squeezing the buffer off the swab too? At first I was only pressing it against the side of the vial, but it's much more effective to squeeze both sides of the vial and squish it on the way out.