In episode The Time of Angels there is a remark about Weeping Angels:
that which holds the image of an angel becomes itself an angel
Is there any reason why in the later episode The God Complex they met the Angels hologram which are impalable by Doctor?
"The image of an angel becomes itself an angel" is a... poetic way of putting it, but it fails to properly illustrate the exact mechanics. That makes sense, however, as that phrase was known to have been written by a madman.
The truth is that a Weeping Angel can project themselves through the image of a Weeping Angel. So it isn't as if a new entity is born whenever you photograph an Angel. It is just that Weeping Angels can use images of their species as extensions of themselves, because "their image is their power," as stated in "The Time of Angels".
The true nature of their ability as a way of projecting themselves was described by the Doctor in "The Time of Angels", not only after reading the madman's book, but also after seeing an Angel do it for himself and therefore having a first-hand observation to work with. To quote:
RIVER: So it was here? That was the Angel?
DOCTOR: That was a projection of the Angel. It's reaching out, getting a good look at us. It's no longer dormant.
So the image of the Angel that came out of the video clip and tried to attack Amy was not a new Angel being born, but rather, it was the already-existing Angel that was being held prisoner aboard the ship reaching out through an image of itself in order to scout out the situation.
Therefore, the Weeping Angels that appeared in "The God Complex" would not manifest, as there are presumably no active Weeping Angels around to project themselves through those images.
