I agree that the iPhone cameras often do not focus, usally in low light or when the camera lens is dirty.
The other thing is that 'touch focus' is a tricky thing and is not as accurate as some might think, camera often cannot estimate the thing that you are trying to focus on and hence takes blurred photos.
I've been using 4S since release and have managed to comeup with some ways to tackle these kind of focus issues:
1) as I said check your lens for dirt smudges or any stuff that may be affecting focusing.
2) iPhone is not a good cameraphone for low light conditions and will not focus as the camera has no LED for assitsting focus, and the flash also doesn't assist in focusing it just lightens up the scene.
3) if unable to focus on an object try focusing something else on a same distance then take the picture of the object you want.
4) some times touch focus just doesnt work for you, if you move the camera or shake it for a while the app will try to focus on centre which helps in a lot of situations.
eg. 'often' when trying to take a photo of a full moon ,when you tap on the moon the screen goes in blur, but if you tap around the moon it focuses on the moon for some reason, i've noticed this a lot.
there maybe more ways to reslove this issue too, i tried to help 😀