New Users Do Not Have Cards in Their Feed
You cannot send cards to users who do not exist. Therefore, by definition, new users will not have any cards in their feed on their first session.
If you want a new user to receive cards, create a Content Cards campaign that is triggered on session start events (with no re-eligibility, so that users only receive the campaign once). They will receive this card on their next session.
Alternatively, you can send the card in their first session; just call
requestContentCardsRefresh on the SDK during that session, a few seconds after calling