Headphones Jack not pluggable into Macbook-pro retina socket
When you insert the headphone into the socket, it makes connections internally against a number of poles which connect to the different rings on the jack end (left audio, right, mic, ground, data etc..).  These are essentially tiny spring loaded contacts which the jack must push aside when inserting, that then rely on sprung pressure to maintain a permanent connection to the ring on the jack.  Here are a couple of pictures, a diagram, and a photo or a replacement part that shows one of the poles:





These poles can jam, get stuck, fail etc etc.  Usually a vigorous insertion is enough to free them, but this is relying on the chamfered edge of the jack to transfer vertical motion into a horizontal movement of the pole.  You could use something slimmer than your jack, and insert (carefully, ensuring you don't insert it too far) before applying sideways pressure directly to the poles to try to get more energy into making them move.