Seems like a similar issue I had with one Clip+ I had. I only had this MP3 player for a few month and I had a 90 degree piece in there so I didn’t have to push in and out using the jack. It does stick out a little this piece.
And this issue is clearly the MP3 player as I tried the headphones on a different mp3 player.
That certainly isn’t the case as it is in the whole way as if I had for 3 month and started have this problem.
I would call SanDisk Tech support about a warranty replacement then. 
