Herr Oberst,
Sorry, I'm not familiar with " Xtracolor enamels". But it sounds (from your description) that not enough of the basic pigment is making it's way onto the brush itself. Assuming you want to stick with enamels for your drybrushing efforts, may I suggest this;
Go to your local art supply store (even Michael's or Hobby Lobby) and pick up a tube of enamel paint in the colour of your choice. It will cost you approx. $3. to $4., but will last you quite awhile. Use this instead of your normally available modeler's enamel. The reason?
You are assured of a proper pigment viscosity in the paint mix. Just remember, proceed slowly. The affect will be quite a bit more pronouned than what you're apparently used to.
Tread.
Failing that........switch to acrylic's. ;-)