Feathering slowly isn't a concrete indicator of sex. I have 4 Isa Brown pullets that are a month old. One we named stumpy because she feathered out so much slower than the other 3. They are sex-linked chicks so I know she is a girl and once her feathers came in, they were the same color as the other girls. Some chicks are just late bloomers. Just be patient. If you bought "pullets" chances are she is just that.