I think sometimes it's not actually intentional - which may be why she said she doesn't know why, and nothing seems to work. I know Sam is younger but he was pretty much potty trained - had the occasional accident but almost never, then suddenly started wetting himself every time he needed a wee again. I spoke to a child behavioural therapist (didn't go and see one but was out for lunch with a friend and her friend, who was one!) and she said that when kids feel that their role in the house is threatened or attention is taken away they can regress to make themselves more needy again - but don't mean to do it. Sam had been fine while Toby was a little baby and he got to play with me while Toby watched, but it happened at almost exactly the time that Toby started to crawl and was suddenly wanting to play with everything that Sam had or be wherever Sam was. All the things that I'd used to potty train him in the first place - rewards, sticker charts and things - didn't help at all and it was so frustrating but he did just grow out of it again x