Daniel Austin, Could be a broken piece off card/dust/debri or damaged inside the card slot mis-aligned lock spring/pin. Use a magnifier and aim a flashlight down the slot and see if there is anything obvious causing the issue. Also try another card if all looks good. If it ends up that the camera needs a new SD memory card slot, it is soldered to the mother board and would be best to have it checked out at a good local camera shop and estimate repair. Links below may be of some help, 1st/2nd link are forums that talks of your issue and 3rd link is video showing what he found to be the problem. Last link is a replacement card reader. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.
http://www.thephotoforum.com/threads/nikon-d90-memory-card-slot-issue.222744/
https://www.flickr.com/groups/867288@N20/discuss/72157626732910540/
https://www.youtube.com/watch?v=QMwO_Y0UO70
http://www.ebay.com/itm/SD-Memory-Card-Slot-Assembly-Replacement-For-Nikon-D3100-D5000-D5100-D90-D7000-/300830924903