Hi and welcome to Android Central Forums.
A couple of simple questions:
1) Does it go in at all? Even a little bit, or does it just go in and stop? Do you get any sound from your headphones when it's in as far as it will go?
2) How long have you had this problem? If you've had the phone for a while and it's just started doing this, you might have some lint or fluff/dirt in the headphone jack. Try gently inserting a toothpick and jiggle it around. See if something is blocking the port.