I've used Winsor & Newton for years with good results.
As you described, having the fluid pull away slightly from the edges of the area being masked
is an inherent quality of using a liquid. It's just the surface tension of the material causing it
to resist being pulled out into small intricate shapes.
Think of a drop of water on a counter or a plate of glass. If you use a toothpick or something
to try and "stretch" it, it tries to go back into the smallest, most compact area possible.
There are a couple of alternatives to try. First, you could use a thicker material. Some
have mentioned using toothpaste or mustard.
But what I usually do is to apply the fluid as closely as I can to my stenciled lines, and
after painting, I peel off the fluid and touch up any areas that I'm not satisfied with using
a very small detail brush.
