It's probably too young to sex then. Usually you can sex, if age is known (usually about 7-9months at earliest) or by 20cm. Some axolotls can take a bit longer to develop; one of our males we thought was a female as he had a small cloaca until we noticed him dropping spermataphore packets round the tank. Are the toe tips starting to darken, if so this means your axolotl is maturing but may not be necessarily sexually mature. It may take a few more months before you'll know.