You don't need a lot of power to do a successful dark slide but the more fun dark slides are when you have plenty of power. The key to the dark slide is timing your kite loop and making sure the kite loop is not too fast. Most guys loop the kite to soon which powers them up and you end up with a hard yank through the air and doing a nice face plant. Do the kite loop at the end of the dark slide just as you start to lose power. This will power you back up and pull you through nicely without a hard yank.
So if your heading is left foot forward you will start to send your kite back to the right ( and sheet in slightly ) Once the kite is at noon fall forward and start to drag the tail of your board (in this case it would be your right foot).
The entire time you are slowly sending your kite to the right ( while slightly sheeting in ). When your kite gets to 2 o'clock is when you want to pull down on the right side of the bar and loop your kite.
I like to loop my kite a bit sooner so I get a powered pop out of my dark slide. It is more fun to me but I warn you if you try this you will eat it hard a few times before you get the timing just right because you will be pretty powered with your loop and it tends to yank you really hard through the air.
Based on your experience it sounds like you simply did not have enough wind. If you are looping the kite and still not getting any power then there was not enough wind to begin with.
Only thing I would add is more on the loop. I do big long darkslide using the 19m & 15m Edge. The key is the loop as stated above. If you are powered you need to do the loop slow and make sure you have drained the power out of the kite before completing the loop or.......
"you will eat it hard a few times before you get the timing just right" lol
One more thing too. I start the dark slide when the kite is still out in front at (going to the left side) 1,2,3 o-clock depending on how much power I have this makes for a nice long slide.
For sure not enough wind.