Are there any conditions where b12 injections would help but oral/sublingual b12 wouldn't???

Not many: It used to be thought that the only mechanism for B12 absorption involved intrinsic factor such that injections were necessary with pernicious anemia or any condition that caused a deficiency in intrinsic factor. However, now it is known that oral B12 in very large doses can be absorbed even in that situation to prevent deficiency. I would guess an absent small bowel would do it.