I am confused. Your profile says u r female, ages 24 yr and you are asking about penis getting erect and not ejaculating.
I hope and feel its a typo and you are a man.
Ejaculation happens when one is satisfied with sex and if the person is involved in the sex.
If you have been having sex in uninvolved manner, ejaculation may not happen
Also if you are a porn watcher, who gets aroused and is used to seeing extreme types of porn to get ejaculation, the sex with your partner may not cause ejaculation.
Next Steps
Leave your penis . Dont masturbate to produce an ejaculation. Masturbate only when you have sexual feelings and an erection together. Just having an erection doesnot mean u r having sexual feelings. Erections also happen in sleep, while passing motions etc
Health Tips
When you are with your partner, imagine as if you are having that extreme sex that you are used to and this may bring about an ejaculation. Also haing sex less frequently can make you ejaculate sooner so keep a suitable gap