Sweet potatoes are notorious for not getting crispy in the same way that white potatoes do. The scientific reason for that has something to do with the different moisture content but we won’t deep dive into that - you’re very welcome - we'll just stick to the practical dimension: how to get the crispiness.
Three main reasons (with solutions) why you’re sweet fries are missing the crunch:
You're missing the magic ingredient...
The guaranteed secret for success to making truly crispy oven-baked sweet potato fries is using cornstarch to give the fries a slightly crunchy exterior. Toss your sliced fries in cornstarch and salt, followed by oil. Play around with various amounts of cornstarch versus olive oil to find the perfect ratio.
Your oven is not 100% preheated...
The most important thing from what I've found, with making anything crispy in the oven, is not to put it in the oven until the oven is 100% preheated. Once the preheating is 100% complete, you pull the pan out, stick the fries on it, then put it back in for 20 minutes. Comes out crispy every time.
Note that the fries do become soft as they sit, so they are best served warm from the oven.
You’re not using the right oven base...
Enter: the cooling rack. Use a cooling rack inside of your pan so the heat/air can get around the fries evenly. It makes a huge difference in the sogginess.
Chop chop, let's try this!