I'm starting to give up on the idea of crispy sweet potatoes. I've tried wedges and they always turn out a bit squishy. I've tried more oil, I've tried less oil, I've tried more salt, I've tried a hotter oven, and there's never any improvement. I've even tried doing ultra-thin slices (pretty much just one step up from crisps) and I've discovered that they still stay squidgy, right up until a magic point when they just turn instantly black...and infuriating, still squidgy.
Possibly deep frying them might make a difference, but I think they must just have too much water in them to go crispy in an oven.