The problem is in Vietnam something like 10% of bombs dropped failed to explode, at all.
Tonnage dropped apparently exceeded 5 million tons (including who knows how many millions of cluster bomblets that were notorious for failing), implying 500,000 tons of unexploded munitions, explaining why to this day people are still being killed/ maimed in Vietnam and Laos....
So I guess the enemy had lots of "dud" bombs to work with...