I've been turned away with a reservation at the ORD Avistar too (on Thanksgiving morning). That was about 10 years ago and I haven't been back since. (Though granted, I've moved away from the Chicago area, but before I moved Avistar probably lost a good $800 worth of my business that I took elsewhere).

In my case they told me they were full, and that my reservation meant nothing to them. (It was not pre-paid in my case...they did not have that option then). At any rate, Avistar's reservation system is meaningless as they appear to take "walk-ups" as long as they have space on the lot.

My advice is to avoid Avistar ORD unless you like to gamble.