Its perfectly normal. You can't night train as such, its totally different. The signal sent to the brain that wakes you when you need the toilet at night is the last to develop and it can take as long as up to age 6. My son is 5.5 and still just does not wake up when he needs the toilet. (He does sleep very deeply though which probably doesn't help). I use well boosted DnFs, Hollow Oak night nappies and Puddlekins night nappies. Day nappies would not cope with his flooding.