Chairs break, and bombs don't always explode.
However, one is more common than the other, such that there is a word for one, and not the other.
A chair that breaks is still just a chair, albeit a broken one. There is no word that means "broken chair".
A bomb that fails to blow up is a dud. Not fake, just a dud. Still a bomb, but common enough to have a name to differentiate it from other bombs.
I read a book about the Unibomber. It took him a fair bit of tries to get it right. One thing the book pointed out is that letter/parcel bombs can are hard to do. It needs to survive being tossed and handled, flipped on its side or upside down, crushed under other packages, bounced around in trucks in hot and cold environments with variable humidity and air pressure, for an indeterminate amount of time, all without blowing up. It can't leak, or smell, or smoke, or tick. And that was all in the days before we had scanners and various detection machinery built into mail processing facilities.
Then it needs to blow up based on an action as simple as opening a box or envelope. It can't be disarmed if the person opens it an unexpected way.
If this was the bomber's first try, they may really just be duds, intended to go of but failing to succeed.