Out of 20 pullets, I had 1 laying at 18 weeks, 2 more at 20 weeks, and 2 more at 23 weeks. Tomorrow is 24 weeks and I have 5 out of 20 laying, or 25%. Of the 5 laying, I have 3 out of 7 Orpingtons and 2 out of 4 Delaware. Some of the others still have pink combs so they are a ways off. Depending on how many you have and somewhat breed dependent, 20 weeks is not a bad age for the first of them to start laying, but some will take longer.
You are probably not doing anything wrong. Your temperatures and hours of daylight could effect it some, but with pullets coming into lay, i would expect you to get some eggs soon. With the time of year, you will probably not get as many as if they were coming into lay in August, but you should get some.
Good luck!