I was thinking maybe the dragonsbane works like a key potion ingredient that needs to be mixed with some other things and some flesh/scale/blood/claw or something from the specific dragon it will be used on. That sort of works on some earlier suggestions that the gradon possesses its own demise. Maybe there are a couple of related approaches. Like you can trick the dragon into consuming food with a good dose of prepotion in it, and by mixing up in the dragons stomach the biological sample of the dragon is incorporated and the potion brews then goes to work. Or someone has to fight the dragon, so a quicker death, thus already complete potion, is needed; therefore, you use potion to poison weapon and then get a realy deep stab/pierce from sword/arrow.