I suspect the egg that exploded may have had a hairline crack. Sometimes an egg will have one that is hard to detect and when you tapped on the crack it exploded. When the chick is nearly fully developed it is pretty much impossible to see any movement. I have tapped on the eggs too. I just leave the unhatched eggs in the incubator an extra couple of days or so. A couple of times I have had chicks hatch that I thought were expired. It was day 25 and I was turning off the incubator and taking the eggs that hadn't hatched out and I heard peeping. The pips were under the eggs so I didn't see them and the chicks were stuck so I helped them hatch and they survived.

