Adaption

Every species other then Icewings, Rainwings, Sandwings and possibly Gemwings are slightly immune to Sandwing venom (hence why Hiddenflame isn't dead) and in this universe, Rainwings' darts have a bit of Sandwing venom and Brightsting cactus juice in them, so that's how they work and why some sandwings are found dead and without a tail barb at the edge of the forest, though their trying to make some with their own venom and a cure that isn't more venom. This means that sleeping darts don't always work on the species immune to Sandwing venom.