I'd say it's both a little normal and a little back luck. The times I've had issues with recreation.gov are when the site is struggling with demand. If timed entry tickets opened up at a certain time, and there were a lot of people trying to claim them all at once, the site often struggles with that.
My previous trips to Mesa Verde, I bought tickets after arriving at the park, but it's been a few years since I was there, and I'm not sure if online tickets were even an option at the time.