Originally Posted by
bajrbajr
It might be that AC doesn't have availability that day
Either that, or AC doesn't even serve BGI that day. Some of these destinations can be a bit more difficult to get simply because schedules aren't great.
I needed specific dates to BGI a couple years ago and ended up having to burn AA miles as a last resort after checking SkyTeam and Star first, both of wihch had either no flights or no availability.