Kwikspell seems to be effective among wizards like Madam Z. Nettles and Warlock D.J. Prod, who have poor magical skills. However, squibs like Argus Filch and Mrs Figg find no results as they have no magical power. Argus Filch had received a letter from them as discovered by Harry while doing detention, but Harry finds out later it will never work on Filch seeing as he's a squib.