Blocks not being placeable ( Per world ???)

Well, to stretch a point, in the new snapshots, certain areas in my world can not have blocks be placed on them. Its in an Iron farm (The Iron Titan), and I cant place blocks on a certain area. Also, upon reloading an earlier world, it turns out I could place blocks on some of the spots I couldn't, but NOT on others. So, its expanding ? There is problem placing blocks. That's all I really know.