LAMB had so much potential for a compelling plot either through the lens of horror, thriller, or overall narrative symbolism, instead they chose the lens of "pretend the hybrid human/sheep monstrosity is totally normal". The movie, in short, lacks any sort of dynamic conflict to move the plot along. Spoilers ahead.
Trailers make it seem like there will be some conflict between the characters, namely between the husband and wife, but there is none at all. The plot could have revolved around the couple disagreeing on the normalcy of raising a hybrid child of unknown origins, but it doesn't. It could have revolved around the couple agreeing to raise the child but being wary of the potentially supernatural repercussions, but it doesn't. The plot ALMOST revolved around a third character who brings reason to an otherwise clinically insane couple harboring what could feasibly be a demon of some sort. Instead, the third character arrives and after a short 15 minutes of timidity accepts the hybrid child as normal. So what conflict is left to move the plot along? The hybrid's sheep mother causing a ruckus by the window.
So now the audience, through the perception of the characters, understands that the hybrid child itself is not mysterious, compelling, or otherwise interesting. The focus of the narrative asks the viewer to divert attention to the motive of the mother sheep and its relation to the child, but the cinematography and direction remain focused on the human character conflict which is NON-EXISTANT (sexual tension when juxtaposed with a half human half sheep child does not even come close to "conflict"). The viewer is left on the edge of their seat, not because the plot is enthralling, but because they don't want to risk missing the moment where the focus is placed back on what they care about...the DAMN DEMON SPAWN CHILD.
After far too much time watching an uneventful love triangle, the plot finally bears fruit by revealing the hybrid sheep father who then kills the family dog, and shortly after the husband, after which the movie abruptly ends. So over the course of an hour and a half we got intense detail and focus placed on the relationship between the human characters, and in the span of 15 minutes the hybrid child's plot is escalated and wrapped up. For a movie titled LAMB the focus was hardly ever on the most compelling character...the lamb. Its a classic case of "Chekhov's half human half sheep child". The audience believes they've bought a ticket to a movie that will explore the origins, motives, and compelling symbolism of a supernatural being, but instead they are given the origins, motives, and mundane undertones of a family that lost a child.
The fundamental issue that makes this movie a flop is that when you include an intriguing character, such as the Lamb child, the rest of the characters should serve as tools to tell the Lamb's story, NOT THE OTHER WAY AROUND. I promise you there are only two types of people who saw this movie: Disappointed people who expected the focus to be placed on the lamb, and people who were satisfied having their expectations subverted by watching a movie that revolved around how parents (both human and sheep) deal with the loss of their children. Subversion for subversions sake does not inherently make an entertaining movie.