I know this has happened to a lot of us flying out of Toronto (and maybe other parts of Canada). In fact there was another thread a couple of months ago about this situation. My situation was also similar...
Back in March, my fiance and I confirmed a group booking through Sunquest (flight and hotel for our November wedding). At that time there were lots of seats available. All of a sudden, in April, my TA called to tell me there were only 9 seats left!! We hadn't even sent out our invites!! I totally freaked out! However, my TA reassured me that this happens all the time, and that Sunquest "always" adds another flight as the date gets closer. So we crossed our fingers, and we went ahead and sent out our invitations as planned, with our original travel info included.
It was super-stressful, but in the end, Sunquest did add another flight and over 200 more seats opened up for us. Apparently this happens all the time...but trust me--I understand that the situation is really frustrating. It's hard to plan when the only answer you have is "MAYBE or USUALLY another flight will be added."
Sorry I can't give you more answers, but at least you know you're not alone...