Technique : Upwind in Light Air
Light Wind Techniques - It may not be you, it might be the wind.
You need to be powered up in order to go up wind effectively. My test before I hit the water on my twin tip is to fly the kite and pull the bar in quick. if it can lift me up, I will have enough power, if it can't, then I will take a bigger board or know going out that I will be walking back. Make sure you power it up on the strap.
Don't Choke! :
If you are pulling too hard on the bar (pulling it in too far, especially with the kite on the upper knots in the back) the kite actually chokes itself out of air. When you pop up from the water, you need to let the bar out or the kite chokes/stalls. I almost always "pump" the bar a few times in low wind to get going. Clear indication of this is seeing the back tips start to fold in and/or feeling the kite "fall back" in the wind window with no power.
If you have heard the phrase "you need to spend money to make money" then this might make sense: sometimes you need to "go down wind to go up wind". Bow kites are masters of generating apparent wind. when you pop up, flatten the board out and go downwind and build speed, once you are moving along, then you can start to edge and the apparent wind will keep you going fast.
When the wind is light, you may need to use an on/off edge to maintain a balance of speed and edging. You can develop a rhythm of "rolling" onto your heels from flat foot. This edge up upwind, the bears off for speed, then edges you back upwind. The harder you edge and press down on your back foot, the further it forces the kite ahead of you and out of the power zone. In light wind, don't edge to hard or you will stall.
If you are rolling on and off edge or able to edge gradually, then turn your shoulders and body to face more upwind. this really helps. If you start to stall, allow yourself to sink into your knees and flatten the board.
Remember, if there isn't enough wind, no one can stay upwind, but it doesn't mean you can't have fun. Do a few runs and walk back up the beach. It's almost always more fun to ride downwind than slog upwind.