Allegiant Air — awful airline - allegiant air!
My daughter attempted to check in an hour before her flight at Stockton, California. She was denied. The requirement was 2 hours in advance. They would not book her on the next flight leaving 2 hours later or put her on stand-by. The Stockton airport is tiny - Allegiant Air would not accommodate the 10 people who were booked on the flight but did not check in precisely 2 hours before departing. The airline was chosen because it's airport location was closest to us - without having to drive 60 miles away from home.