Is it normal to have some discomfort in completely retracting the prepuce after a full erection?

No: The male prepuce should be easily and fully retractile by puberty. Discomfort can come as result of some degree of phimosis, a tight frenum or bablanitis (inflammation of the glans and/or foreskin. All three require treatment. Any good urologist can help with all three, a primary care md or do can help with bablanitis.

Urologist: Its not normal. You could be dealing with early phimosis. See a urologist for evaluation and advice. Early treatment for phimosis is advisable.