This may be a long shot, but try holding off on crafting your next purple wisp right away, if possible, keep a couple in inventory until the mines are ready again. I was crafting the potion as soon as the wisps dropped and I had several collections with no wisps. I decided to try to collect a few before crafting more, and lo and behold, the drops rates improved once I had some already in my inventory. It may just be the random luck thing, but it seems almost as if the game only gives what it thinks we don't need. :-/
I really wish I could share some purple wisps and lucky dust. The idea for a trading post in this game is an excellent one!