Son keeps getting denied on Chase & Citi cards

Background: my son, 20 yo, in college but have part time job making around 15-20k/year. Since he is a teen, I have put him on my additional cards when I churn chase or citi cards & add him on my amex cards. When he’s 18, he’s able to get a barclay aviator MC & Amex hilton (no annual fee version). There are activities on those 2 cards & paid in full every month. No negative remarks, no late history. His current credit score is 760+.

We have tried to apply him on chase hyatt & citi AA. However both are rejected for insufficient credit history. He has 2 years of credit history on his own cards. Do chase or citi has more stringent requirement for those cards? What citi or chase are easier to get so he can start building his history with them? Thanks