Welcome to the wonderful world of chickens! Do you know for SURE they are roosters? It is often impossible to tell when they are young. If male, they should start soon, gradually working up by making squacking noises at first, then getting louder. Some may start sooner then others. I have 6 roosters at the moment, they are all a year old, and I still have yet to hear two of them crow, if they ever will. Also, be sure that when the birds and the bees issue starts in a few months between the hens and roosters, that the roos arent too hard on the hens (they can get feathers ripped out and bleed if there are not enough hens per rooster). Perhaps some of your others are girls which may solve that! And dont worry, youll hear crowing soon enough....and it will NEVER stop!