trouble washing

Just to clarify a bit here, a dull coat is dull because there's lots more micrscopic pits in the paint when it drys, reflecting light all different ways, thus the dull appearance. A gloss coat is mouch smoother, thus giving the shing appearance. Hence the easy flow of the wash.

Yup, like everyone said, it has to be a gloss finish. Unless you just want to see the wash propagate through the texture of the flat finish (don't laugh, I understand some people actually do want that). If you want to keep the wash confined to an area such as a panel line or recess the surface MUST be smooth and not flat.

You put the gloss coat on first, before weathering.
After weathering you apply a matt coat. Model done!

Do you have to use a gloss coat? What if your doing a WWII era fighter and need a dull finish?

You could also use a regular pencil to fill the panel lines. You don't have to wait for anything to dry and if you use a gloss finish first you can use Windex to erase any mistakes or even use an eraser for that matter.

Doom this is a simple thing to fix. You must apply a clear GLOSS coat over your finished paintwork before you start any weathering or washing panel lines. The gloss coat is up to you but a lot of people (me included) use Future floor polish, however if this isn't available something like Tamiya clear gloss or Gunze clear gloss is just fine.
The gloss coat acts as a barrier and protects your paintwork and allows the wash to flow smoothly down the panel lines. I use MM enamels followed by a Future gloss coat, followed by an artists oils wash. I wipe the residue of my wash away with a q tip lightly dipped in thinner after a couple of minutes, then I use Pollyscale clear flat to finish up.

Cheers
Darren