If you miss your Allegiant flight or fail to check in on time, your ticket is usually forfeited with no refund. The only exception is if you purchased Trip Flex, which lets you make a one-time change or cancel up to one hour before departure. To avoid being marked as a no-show, complete check-in early and arrive at the gate well before boarding closes. Need help with Allegiant Airlines missed flight policy? Dial 18005944991 for...