It could be any number of reasons - I have three hens that are 1 1/2 years old, and 7 pullets that are at the point of lay. They were not combined until recently, but 2 of the 3 hens have stopped laying. One is obviously molting. The other still looks nice and fluffy, but no eggs. My RIR hen is still laying faithfully - 4 to 6 eggs per week. Barring other complications, it may simply be the shorter days that are affecting the egg laying cycle. They are a bit young to be molting, but it is possible! I doubt that it's the introduction of your chicks, other than the normal stress of integrating with new birds.