they all do stuff at different rates, remember that
my eldest ds was slightly later with learning his days of the week but knew all kinds of stuff that ds2 and dd who knew days etc earlier, didn't. Developing different skills at different rates is fine. Another one - my boys just did not do writing early, ds2 read at just 4 but no sign of bothering with writing anything. DD at 3 wants to write her own name and all kinds of stylised fancy "letters" (unfortunately not all of them English lol, several appear to be nearer Greek) despite me having in many ways much less time to do concentrated stuff with her.