That's what you can't always do, if you buy your accs.
They are just fine until a certain point, then they get flagged and eventually banned.
So it's not always the end user to blame. Even if you do everything right, the accs can be doomed from creation.
For instance, the accs were created from not exactly the same ip addresses, but from the same c-block. If for any reason that particular c-block gets blacklisted, all the accs which were created from it are gone, so you don't know for sure what hit you, you only notice that your accs are banned.
The account creation is a crucial step on every social network, if something is not right with it, there's almost no point doing anything with the accs, at least for long term, because they can be gone anytime.
I'm not well-versed in FB, but adding 20 friends daily doesn't seem extreme too me which should result in a ban, this alone cannot be blamed. There must be something else, which can be what you said, common things between accs, can be the accs themselves, can be ip/proxy issue etc.
So my point is that you can't always find the reason, no matter how hard you try or how skilled you are.
That's when you just dust yourself off (maybe change account provider) and try it with a new batch of accs hopefully with better results.
