No, I am not going to pull an Eleven11, mainly because I don't have a backup plan.
Anyhow, lets say for mechanical reasons, the altimeter that triggered the detonation fuses fails. So the B-29 pilots are dissapointed... for about 5-10 seconds.
The Little Boy gun-type bomb design is a highly dangerous one. It is the only nuclear bomb design I know of that can go off by itself. So when the bomb drives itself into the ground, the uranium 'bullet' is driven into the sphere and we get a critical mass.
And the bomb goes off.
Now, in OTL the bomb left no crater cause the fireball never touched the ground, but here the fireball digs a 500 yard (457 meters for you foreigners) diameter crater into the ground.
Now, the initial damage is going to be less then OTL, because the thermal pulse and shockwave is semi-blocked by the ground. There is still significant damage and a massive firestorm, but that's not the problem.
The problem is all that dirt that was vaporised has been turned into radioactive particulate matter and is now being blown over Hiroshima. Thus, the result is a significantly more 'dirty' then explosion then OTL.
Mainly, I am interested in how this effects the local ecology. How much longer does it take before Hiroshima can be rebuilt? Or do the Allies decide to ignore the pesky radiation and force reconstruction forward? Or what?