That sounds like a nightmare right before an event. Did the foam feel different when you first pressed a stem into it, like was it too soft or crumbly right out of the box? I've had cheap foam just turn to dust when you try to push anything woody through it. What kind of flowers were you using that wouldn't stay put?
Honestly, I wonder if the foam got wet at some point before you even used it! A damp box in storage or a humid garage can totally wreck the structure before you open it. It might have looked fine but lost all its grip. I've seen that happen with roses and heavier stems like lilac, they just lean over and won't stand up straight.