This is normal, when new birds are introduced into the flock. Chickens don't like outsiders, and it can take them a VERY long time to fully accept new members into the flock, without hanging out as separate micro-flocks. This can also be complicated further if the two groups are of very different breeds, sizes or looks. Are the older chickens silkies, or normal breeds? If they are not silkies, then there's even more reason for the two flocks not to mix. Chickens are racist and don't like those who look too different. So the older flock may be refusing to accept the new flock for multiple reasons - newness, age, and looks.
In my current flock, I have 3 generations of chickens. Generation #2 was hand raised and introduced at 1 month old, and generation #3 was hatched and raised with the flock. Generation #2 has been with the flock for almost 2 years now, and is the same breed as the majority of the flock, yet they still aren't fully accepted as equals. They all eat, sleep and hang out together at this point, but at every opportunity, the older ones make a point to show the younger ones how much they dislike them - they chase them around, don't let them near when there's food or treats being given out, and maintain a wide radius of personal space with them that they don't with each other. Generation #3 hatched and grew up with the flock and is the same breed, but is enjoying absolutely no benefits from that, and instead is everybody's b**ch, almost a year later
So yeah, "one flock" is a very subjective term. As long as there's no blood and everybody is able to eat, drink, lay and sleep, then it's as good as it gets.