Maisie Williams is a good actress but the film is slow moving and almost nothing really happens other than closed minded people not wanting to talk about the realities of life.
That being said, there is an actually problem and it's not with the film itself but rather with those that have watched it and how clueless the conclusions seem to be.
1. The reason Lydia is acting the way she is is obvious. Because her semi lesbian relationship was in the process of disintegration when the other girl died after cheating on her. She doesn't know how to deal with it so she begins acting out. She is, after all, only a child.
2. The fainting spells are also rather obvious. They are real for some of the girls and not for others. They have been picking at the wall in one of the rooms. Obviously there must be mold or something in the crack. It could also be lead poisoning given the time period. And then they are sticking their fingers into each others mouths or their own. It's quite obvious that some of them have ingested something that will harm them.
3. Not all of them die, so there is another issue. Abigail was pregnant. There is a blood toxicity that can happen while pregnant. When I was little my best friends mum was confined to bed for months while she was pregnant because this almost killed her. She ended up in the hospital for almost a month. This, combined with what might be in that wall, is likely the cause of Abigails death. Given that technology wasn't nearly as sophisticated at the time this movie takes place, they may not have known what caused it.
The movie may be boring but thinking through the answers shouldn't be so difficult.
