Hi Brenan: First off,....you do not have Phimosis. True clinical phimosis is when the foreskin is so tight, you cannot retract enough to see the urethral opening.
I think you mean something different when you say the foreskin does not roll down while erect.
The other respondents have taken it as a tight foreskin question.
I think what you mean is the foreskin is long enough, it still covers the glans while erect.
Some men have short foreskins that retract behind the rim of the glans while erect, and other men have long, prominent foreskins that can be manually retracted.
The rest of the men who are fortunate enough to be intact (not circumcised) are somewhere in between, and all are normal.
If you have a long, prominent foreskin, that means you have more nerve innervated tissue to provide stimulation and pleasure,....for both partners.
You should silently thank your parents each day you are alive for having the wisdom to leave you whole, complete, intact, normal, natural, and not allow some hack in a lab coat to mutilate your genitals before you were even out of the delivery room.
If I am way off base here and the question is being asked because the foreskin is too tight,.....there is nothing wrong with you that some simple foreskin stretches cannot solve.
You have a naturally tight foreskin, just like every other intact (not circumcised) man on the planet had at your age.
Over 85% of the males on earth are intact so you are on the winning side. The other 15% are envious and wish they still were intact.
Follow the information in the links below and work the foreskin stretches into the daily shower routine.
Let the warm water loosen and relax the skin before stretching.
Stretches are to be firm, but gentle, with no pain. Pain is a damage indicator. Over stretching till it hurts can create micro tears in the skin opening that will heal over with non elastic scar tissue, making retraction more difficult. No pain,...ever.
What will happen is the Frenar Band (google it) of muscle that closes down the skin opening will relax and expand enough to allow the foreskin to glide effortlessly back and forth over the coronal rim of the glans.
You should see some good results in two weeks, with full and painless retraction in 3-4 months. If it takes longer just keep stretching.
Do not listen to anyone who claims you need to force the skin back to clean under there. Nature has this well in hand. The foreskin/glans combination produce a powerful anti microbial compound called Langerin. Langerin wards off bacteria and keeps this area clean and disease free.....just like you are now.
Go to: www.norm.org and read about the reason and function of the foreskin. You will see how very fortunate you are to still have yours, and how misguided are those who promote circumcision for any reason.
Good luck and I hope this helped.
Mithras