Have you held them and looked at their wings. The wings will be a different shape. A girls wing will go in or have shorter feather near body. I believe there is a post on one of these pages about it. I found it very helpful at this young age. Even some pullets will have small spurs at this age.
Wing feather sexing is a sex linked trait and, as such, must be specifically bred for with a carefully chosen cross between a fast feathering male breed and a slow feathering female breed -- the resulting chicks will have the feathering rate of the opposite sexed parent and the "reading" must be done within the first three days. It is not a reliable or accurate method to sex any other chicks.