When my DS was about 12months i bought him a different brand of dummy (one of those ones that glows in the dark so you can find it at night) and he hated it! He kept spitting it out.. i guess it felt different in his mouth. I'd thrown his usual ones out because they were getting grubby and old so i didn't have anything else to give him at the time. I didn't get around to buying anymore, i kept offering him the new one but he would just pull it out of his mouth and look at it... so over about 2-3 days he just weaned himself off it. Maybe try using a different brand/kind and see if it works?
Good luck