OK, totally screwed this up causing the pax to wait an extra three hours. First, kudos to AS for a great system but there was no six hour window. Every flight was open for the whole day and the early flight filled because I did not attempt until six hours prior. Wish someone would have given me a heads up but maybe everyone thought I would be attempting at 10pm. Should have checked it after midnight rather than waking up at 4:30am. No issues with AS as the system is so passenger friendly that you have to be vigilant to get what you want.