Kajo said..
I also find it impossible to use the depower on those lines, I don't seem to have enough strength to pull it in if I need to.
Any thoughts?
Problem comes from too much tension through the clam cleat jamming your trim line in the teeth.
Simple solution is to take the tension off by grabbing the front lines above the bar for a second or 2 while pulling the trim line out of the cleat.
You will only have your hands off the bar for a moment & pulling in the front lines will depower your kite while you are doing it anyhow.
May be awkward for beginners the first couple of times but its as simple as once tried.