You are making the same mistake I, and I'm sure many others, make when we first try out the old wash technique. Not protecting the colour coats. If you paint with enamel paints, then use enamel thinner in your wash it will lift the paint. you need to protect your colour coats from the effect of the thinner. When I first encounted this problem I switched to artists watercolour paint for the wash. This solved the problem, but I wasn't able to get the effects that I wanted. The answer is to use a clear varnish of some type. Enamal varnish is no good, it will lift aswell. The answer is to use Klear (Future in America) floor polish, yep floor polish (it's great for canopies as well but thats another story). Here's what to do:
I'll start from after the colour coats are on, (My weathering process starts before then, if you are interested let me know).
1. Give the paint plenty of time to cure (dry properly), each manufacturers paint is different, so between 24 hours and a week. (Xtracolour definatly a week).
2. Remove all dust and particles.
3. Brush on a coat of Future. I use a soft 1/2- inch flat brush. Transfer the Future into a small container, I use a film canister. Dip in the brush and draw it over the edge of the canister, removing most of the Future, apply to the painted model. Gently does it, to much future, or being to aggressive will cause bubbles. These may build up at edges and detail anyway, if they do, dry out your brush on a paper tissue and touch it to the bubbles, or draw it through the bubbles to clear them. Future is self levelling and dries quickley. Clean your brush with plenty of water.
4. Apply a second coat of future. Wait at least 30 mins, I wait an hour for the first coat to dry.
5. Apply your decals. ( The first 2 coates of Future act as your gloss coat as well).
6. Apply a third coat of Future. This might not be necessary, depends on the decals.
WASH TIME:
First of all a few things to think about:
a. Colour of the wash. I would avoid black, to stark. I use a mix, depending on the camouflage colours, for example, a MK V Spitfire in Ocean Grey, Medium Sea Grey and Dark Green, I would use Madder brown, Black, Paines Grey and white, for the top surface with more white for the undersurface. These are artists oil paints. You can use enamel paints, experiment.
b. Thinner, any is fine, I use the cheapest white spirit I can find.
c. Consistency. Again experiment. My washes are relatively thick, because of what I'm trying to achieve. Basically you want very thin paint.
THIS IS WHAT YOU DO:
1. Mix the wash in a small container, I use caps off milk bottles.
2. Use smallish brush, dip it into the wash.
3. Touch the brush to a panel line and capillary action will draw the wash down the line. The same thing will happen around raised detail. Where it stops apply more wash.
4. When the thinner evaporates it leaves the colour in the recesses and around raised detail. There will be areas that don't have enough colour, you can reapply the wash. Areas like rudder and elevator hinges lines require more anyway (and probably a bit darker).
5. It's up to you when you wipe of the excess. If you don't leave it long enough you will wipe away the colour as well. I leave mine on for up to 24 hours or more.
6. Wiping off the excess. Take a lint free clothe, wrap it around your finger, dip it in clean thinner, dab off the excess thinner on another clothe. Wipe over the model, in the direction of the wind flow. As you wipe off the excess use a clean area of clothe, any stubborn areas use abit more thinners. In hard to reach areas use a cotton bud.
7. Apply the matt varnish of your choice.

There may be other issues that you need to think about, before, during or after the wash, to do with weathering but thats a different subject.

I think I've covered everything. Don't hesitate to contact me again, If I've not made sense or there is anything else you would like to know