Even when the bombs don't penetrate the concrete, the shock waves are disabling to the inhabitants. Since the bunker is in a fixed position, once the artillery is zeroed in on it, you just pound it till it breaks, and there's nothing the inhabitants can do about it. Ground attack airplanes firing at the gun ports, enveloping the bunker with napalm, phosphor bombs, etc., all are effective and fairly cheap to do.

For accounts of this by survivors,

"D Day Through German Eyes" by Eckhertz