This can come later on in puberty. Eventually you will start having "wet" dreams and you will notice it standing up. Sometimes it's delayed. If you have shown all the other signs of puberty (pubic hair growth, deepening of voice, facial hair growth, under arm hair) and you cannot achieve an erection by stimulating the penis then you may want to talk to your pediatrician. On the other hand if you have not shown all of the signs of puberty listed above then it's okay, don't worry about it, that would mean you are still developing.
maybe it's still developing , u r still too young and in your puberty , it will grow bigger soon and hopefully gets erected better