Make sure you are keeping the new ones separate for anywhere from a few days to a week or so to make sure they don't introduce any unpleasantness to the tank.
I've been floating my new girls in their cups in the big aquarium for the last couple days of their QT. This lets them see their new home without their new flatmates being able to get TOO up in their faces. *so far* this has worked great, by the time they are released, they have been thoroughly inspected by the existing girls. I've actually had a funny problem on the last couple of releases...I turn the cup gently on its side to let the new girl swim out. Which is fine, I then have to wait for up to 4 existing girls who swam IN to determine for themselves that there's nothing exciting in the cup
All of my sorority girls are chain purchases, and yes, I did lose multiple ones either before or shortly after release (None were issues with the other girls though, at least), but as someone else said, just because they come from a chain and ore often in a fragile starting state doesn't mean they shouldn't have a good home.
Keep at it, you'll arrive at a stable group, and when you get there, it's great!