If your first attack is the forward smash the glass shouldn't break. The glass takes some "wearing down" before the bag goes flying through it.
So the reason your other characters are able to get it through is probably because of two things:
1) You had already weakened the glass prior to smashing it with your other characters.
2) You smashed the bag when the timer was at 1 (the glass completely disappears when the timer is at 1 allowing you to smash the bag).
It's nothing out of the ordinary.