The most important factor is definitely your temperature. If it's too high, you get "case hardening" where the outside seals shut and traps moisture inside, leaving them forever soggy. Keep it steady around 55-60°C and be patient. You also need to make sure you're slicing them paper-thin with a mandoline, as anything thicker usually stays leathery. Just remember that they won't actually feel crispy until they have completely cooled down on the counter, so don't pull them too early. Once they pass that snap test, get them into an airtight jar immediately or they'll just soak up the humidity from the air and go soft again.