Why has it gotten harder to rinse all the soap out of my dreads? - Dreadlocks FAQ's
As the dreads really begin to lock and tighten, rinsing them does become more challenging. You'll have to rinse them like a sponge, allowing them to fill with water, then squeezing it out. The dreadhead soap is unique in that when you rinse it away there is nothing left behind. This makes it easier to rinse that other shampoos, but even so, if you work the soap directly into the dreads you will need to spend some time squeezing them and rinsing them. Fortunately for day to day, regular washing this isn't necessary. You can concentrate your shampooing and scrubbing on the scalp. The amount of warm soapy water that runs though the dreads is while you wash the scalp is sufficient to keep them clean. Since the soap that runs through them is more diluted it rinses out much quicker.
If it continues to take a long time to rinse the shampoo out completely you may be using too much shampoo, try using a little less. Because dreadlocks make moving lather from one part of your head to another more difficult, it's usually easier to apply the shampoo in small amounts to multiple areas rather than applying all the shampoo in one area and trying to spread it around. Applying shampoo to the top and front -- then the back -- and then the sides, seems to work really well. =]