And, in the strictest sense, immigration is not multiculturalism. looked at closely, the whole world is multi-cultural, there isn't a society out there that isn't a mixture of several older ones. As I said earlier, from the point of view of those at ground level, there is always strife, be it Normans and Saxons, the various nation-states of Ancient Greece, the Roman Empire etc.
Given several hundred years, those cultures, with disparate languages and practices have merged together into something new and greater than the parts, but that integration always comes with problems and strife for those involved.
I wouldn't say multi-culturalism has failed, but it is hard and we have to work to make it successfull, on both sides. It will never, ever be easy. We live in a world where we expect results instantly, and some things, like the growth of societies, simply cannot happen on the time scales that we have grown accustomed to.