Unfortunately, there's no pity system of any kind, so theoretically you could continue not getting anything forever if you're simply unlucky. It's pure luck and there's nothing you can do to influence the drop rate afaik, so you just gotta keep going at it until you get all you need.
One thing mentioned before and in other threads, in case you didn't know, is that the color of demiatma dropped depends on where in the map your FATE/CE is located. So if you already have all you need from one color, you can ignore that area if you want.
