Consistency is really the key, especially for that first year of life.
If you were to tame them consistently for the first year, then ignored them for a few months, they would regress and lose their tame-ness. (This is true for any animal, even cats and dogs.)
But once they've been tamed, it's always much easier to return them to that state.
The other birds can have a huge impact too. If they're with a bunch of flighty, super frightened birds, then they're apt to be the same. (Which is totally understandable. If you walked into a room and saw two people looking absolutely terrified and acting very scared of something, you'd likely feed off of that emotion and start feeling the same way, even though you haven't actually experienced anything frightening.)
The trick is to ensure that they view interactions with you as rewarding. Defining "rewarding" for the individual is the key, really. Some view the actual interaction and attention as inherently rewarding.
afinch had a really good example of this in the video where he was rubbing their cheeks and they were clearly loving it.
Others are more food-motivated.
With most, you use a combination of the two.
Personally, I only work long term with the birds who find human interactions inherently rewarding - the ones who enjoy the interaction.
If they don't find you enjoyable and they're only in it for the food, then it's just.....just not as rewarding, I guess?
I don't want to "pay" a bird to be my friend. So if you don't want to be my friend, I respect that. That's perfectly okay.
For those guys, I just work to maintain them in a semi-tame state. It just makes life a bit more enjoyable and less stressful for them when they're not terrified of you.
afinch have you tried using a clicker?
That would be fun. "Click, Click" and a flock of birds comes zipping toward you!
Ha!
I've gotta try that.