For most collections, when a collection is first released, not all cards are released at once, and not all cards go into the same market for purchase.
For a typical collection, cards will first spend some time in the premium markets (requiring a subscription, or paid tickets, to use) before migrating into the completely free credit market.
So if you can't find a particular card in the free credit markets for a typical active set, it simply hasn't migrated to that market yet; you can either wait for it to migrate there, purchase tickets and buy the card from a ticket market, see if it's for sale in the Fort Worth auction market, or see if it's for sale in the PackRat Facebook Group.
*** Note: Not every set in the game will have cards that are eventually available for free in credit markets or the Rats. For some special sets, like the 400th, 500th, 600th and the 10 Year Anniversary Rat Pack sets, cards won't end up in credit markets. These sets are rare, special, require retired cards, and are structured differently and so market movements are different or nonexistent. ***
A retired card (part of a retired collection) can only be found and purchased in the ticket markets, unless it's being sold in the Fort Worth or Moscow auction market (or on the Facebook group page).