What were some of the things that went wrong with the nusery in "The Veldt"?
Your question implies that there was some kind of technical fault that made the nursery malfunction. I would actually disagree with this. I personally think that there was nothing wrong with the functioning of the nursery at all. It did its job brilliantly, and very convincingly, as the introduction to this story shows when George and Lydia are terrified by the reality of the sight that they see. The problem with the nursery lies not in how it worked and the mechanics, but in how it was used and the unfettered access that the children were given to it. Consider the following quote from McClean, the psychologist who comes to take a look at the nursery:
You've let this room and this house replace you and your wife in your children's affections. This room is their mother and father, far more important in their lives than their real parents.
The real problem with the nursery therefore is not anything to do with its mechanics but in the way that the Hadleys have allowed it to become so important in their childrens' lives that it is now their surrogate parent and a far more important force and influence than their real parents. This of course only serves to cement Bradbury's central message about the dangers of technology.