New baby

Good evening all. Today I picked up my newest baby. Although fully weaned Sydney is very young. I have gotten two other cockatiels before this baby. Both are sweet as pie from the minute they came home. However this baby is terrified. All were handfed. I can’t understand why ? I went to visit several times over the last 5 weeks and the baby was fine. But now when I brought them home. Sydney wants nothing to do with me, even hissed at me ! That’s a first for me and I’m at a loss. Where do I go from here ? Is this a personality issue or a socialization issue ? Again both of my other tiels are from the same place.