As it has been observed in many other cases. In the Enfield Poltergeist case as well as the "Lindley Street" case, the girls were caught hoaxing events. Like Maurice Grosse, one of the SPR researcher in the Enfield case, the researchers in the Lindley Street case came to the conclusion that the girls liked being the center of attention, and when public interest started to wane, they tried to either provoke real phenomena or hoaxed them, so that the people wouldn't leave and the media wouldn't give up the case.
Even if there's psychological problems to be considered with the adolescent girls, I don't think that we can just dismiss the credible witness reports. There is a written affidavit by a policewoman in the Enfield case, who saw a chair move on its own, without anyone near it. She didn't find any "strings attached". And if you read "The world's most haunted house", it has the original statements of policemen and firemen who saw TV sets and fridges being moved, again with nobody near them and no sign of trickery or natural cause to be found.