I had the exact same issue before! I read about using the same technique and just couldn't get it to work for me at all. Very disappointed... :(
I tried with less soap and didn't get any better luck. I finally took the model in the house and ran it under the faucet, let it mostly dry and tried again with better results.
I always use enamels for my washes now btw. I generally use water-based acrylics for the main color and dry-brushing so I use enamels for the wash so that I don't have any issues about the paint coming off.