Jeff: Did you give the whole door latch assembly a GOOD looking at when you changed them out? Everything look the same????? If everything was the same what about a good lubing into the tumblers on the lock itself, maybe some crap or a dead bug in there keeping the key from turning in the lock direction.
Most of the locks I have seen have those tiny little cylinders that fit the key, something could have locked one up, no pun intended.
Some areas of the country seem to be harder on lock sets than others, humidity, salt, dust, whatever. I know in Fla. If you don't use that lock for a while it feels like it's full of grit. Ya never know if it will work before the key breaks. Always use a premium grade lock lube for in there. Dry type if you can get it.
Keep us posted.