Phil,
I just had the same problem and had started a thread about it, perhaps you saw it:
www.seabreeze.com.au/forums/Kitesurfing/Newbies-Tips-Tricks/After-power-stroke-its-not-moving/ Your problems will be solved if you go out in slightly stronger wind. Also going downwind a bit further before starting to edge helped me to pick up some speed.
The kite needs a lot less power to pull you once you don't accelerate anymore but just keep the same speed.
It's the same with a car, accelerating it requires much more fuel (i.e. power) than keeping it at an even speed.
Finally, as already been pointed out, newbies (myself included) make the mistake of sheeting in thinking it gives more power permanently. It only does so temporarily and then stalls the kite if not sheeted out again. So for the power stroke sheet in, then as you bring the kite back up, sheet out. If you need another power stroke fly another sine from the top and sheet in, then sheet out again as you fly the kite back up.
Hope this helps, as I say I just learned this myself so we're probably on a similar level...